https://www.bbc.com/news/world-us-canada-47520029
How the hell would that dude just happen to come across such a tape and have no clue where it came from?!
If he rents or had bought the house and the old owners had left stuff behind, or even bought a box of random tapes at some point, at one time you could buy random tapes and discs quite cheaply to try and reuse or because a label might suggest it had some good software or films on (there was also a market for second hand record able tapes for distribution of things like TV episodes*).
Don't forget "storage locker" clearance is also a thing in the US, so he may have got boxes from that which he's not gone through, or the previous owner/renter of the property might have done.
It sounds odd but I'm going to guess he's not quite so stupid as to go to the police with underage porn he's found if he's got a collection of it, so there are probably details that aren't getting reported but that the police will likely have checked up on/his lawyers have checked out to make sure he's acting legally.
It would be an absolutely huge coincidence though, but then again all sorts of oddities have turned up in rental properties and house/storage clearances.
*I remember at some of the computer fairs, and a lot of the comic fairs there used to be that one stall selling things like episodes of the current season of Buffy, or Star Trek recorded from the US broadcasts months before we got them here (often on tapes that had obviously been used before).