Is this recruiter/manager trying to steal the fruits of my labor or am I just paranoid?

None, this looks like a scam.

Under no circumstances, you are supposed to give away the exact minute details of the working project for your current employer, let alone the source code and/or documentation. That would be a serious violation of the contract in most of the countries and most likely you can end up facing criminal charges. No reasonable employer would even ask this of you (or any applicant).

If they are willing to check "your skills" thoroughly, they can ask and arrange for an on-line or in-person coding test, there is nothing in your existing code that proves your skills. Heck, you might even showcase code written by someone else as yours, so there's nothing that can be proved by either sharing or receiving the code for your current employer.

Stop communicating. Run fast and run away from this.

If you currently work for a government contractor, report this to your security group

This sounds like attempted industrial espionage and/or traditional espionage. If you work or have worked in any way for the government, directly or indirectly, report this to your security rep immediately.

Otherwise, run away as fast as you can

You could get into legal hot water by participating in what appears to be industrial espionage. Run away! If you have concerns that you may have already crossed legal boundaries by violating IP law or NDAs, contact a lawyer.

He mentioned how a similarly experienced developer had submitted 20 pages of such information and was asked for more. This is apparently an application requirement set by the position's manager. I said that it sounded like a lot of work but could get it to him sometime the next week.

This might sound crazy but I wouldn't be surprised if the guy was some sort of state ran operative. It sounds like he's soliciting people with certain profiles to determine things. A 20 page technical detail sounds insane. Would you give 20 pages detailing everything you did to a stranger on a street just because they said they work as a recruiter for some company? Why would you trust a person on the internet asking for the same thing as someone would as a stranger on the streets?

I wouldn't even reply. Just delete and continue on.

I would strongly recommend that you inform your previous employers about this recruiter, because this is most likely an attempt at company espionage. Technical details like this are none of his business. If he says that someone submitted 20 pages of such documentation, then the recruiter is frankly lying. And I have never, ever encountered a prospective employer who was interested in that kind of information - actually, most wouldn’t want to know that kind of thing because of fear of legal consequences.

I'm from an Engineering background, but it would definitely raise a few flags for me. The only way I would even consider showing anything is in a personal meeting, printed on paper, and take it back when I leave.

Don't send anything, as you might be delivering company secrets to a competitor. Disclosing any sensitive information should be done only after getting an NDA signed.

I would question any additional request from this so-called recruiter.

As best as I can guess, one of two things is happening here, neither of which is good:

1) The company is asking you for reams and reams of data just because they can. They want to see if you'll do it, knowing full well that no real hiring manager will actually read the entire body of your corpus (plus, it's code not prose, so it's much more difficult to read). They're putting you through the paces to see how well you acclimatize to ridiculous requests. This means that you can be expected, on the job, to field such ridiculous requests and expected to do them. This is not good.

2) The company has such few applicants that the hiring manager has the spare time to read reams and reams of code from applicants to determine the good ones. This is a job nobody wants, and for good reason (because they ask for your entire corpus of everything you've ever done!). If nobody else wants to be associated with this company, why should you? Furthermore, it also speaks to the level of micromanagement in the company, where the hiring manager wants to read every line of code you're ever going to write, and has nothing better to do with their time than critique every member of their team personally and individually on a line-by-line basis. Run away from this type of micromanagement.

In any case, submit what you think is reasonable. Do not submit any code you have written for any employer, that's almost certainly very illegal. Take like 15 minutes of your time and put together a set of projects you have completed that you would feel comfortable submitting and ship it to them as a portfolio. If that's not enough for them, then simply say "sorry, that's all I have for you, if that's not enough then I think we're done here".

For future, you might want to consider hosting your personal projects on GitHub so that anyone can easily peruse your portfolio at their leisure rather than putting in this much work. It's near-zero work for you to put together a GitHub account, and then you don't have to go through this. Simply say "Look at my GitHub" and be done with it.