> or maybe more likely a previous developer tossed him a (very nice) bone.

This is what I am thinking. It seems a long shot the source code was simply discovered at a garage sale by chance. Too much coincidence that a. the THQ dev still had the code just kicking around and b. a computer savvy person happened upon this garage sale, buys said artifact and discovers the code. Stars must have been aligned.

I think the THQ dev likely knew the "buyer" or someone put them in touch and the cover story is "oops, I accidentally sold the source at a garage sale" in case lawyers get involved.