OK, that would be a weird thing to make possible. 100 most popular games would legitimately include the earliest releases to the latest, I think, which would not be hardware related....so this screams "made available in the digital store to play on PS5." If that is the case, then that is not BC.
BC = your PS4 disc purchased in 2015 or whenever can be played on the PS5. Just like all the other discs from your PS4. That's what BC means. The end.
If what you're saying is true, then it sounds like they are just selecting some titles to make digital and "accessible" by the PS5. ...now, if they are talking the top 100 "disks/games" that were popular, meaning the physical disc that you got in 2015 out of their top 100 can be played, but some other one from that year can not, then that would be insane....could they do that with some sort of software key that allows it to work? Well, of course they could...I'm not suggesting that is what's happening. Very unlikely.
...but I mean, it is Sony, right?
FWIW, I specifically picked up my PS3 60gb, among the 2 versions that were available at the time, I think, because it was full BC. I could play all my PS2 and all my PS1 games on it. ...I still can, and it still works, lol. (then again, maybe it was the only one around at the time, so it was just known that "PS3 is full BC," and well, we know that certainly changed with all the following releases.)