AFAIK "numbers matching" simply means it's the original engine that shipped bolted to that chassis and body from the factory. It certainly can be removed and rebuilt / modded then put back in. The SNs would still match up with factory records. I don't think that's what's being described here - it could be described better as "fraud". The guy openly states that he stamped the block with a "correct" VIN. Probably illegal, certainly unethical and definitely odd.