Yeah, maybe not a cut and dry 315, but if you got the strength and endurance to do 235 for 5x5 then it's not a stretch to assume you could do 235 for about 8 which roughly translates to about 290 which is fairly close to 300.
I don't even think buddy in the video who weighed 230 got to 300 lb in that whole year and a half of training.
I didn't watch his whole video. But number 1 that's a lot of "rounding up”. Number 2 not everyone is trying to go up forever. I got to 2 plates and i just said fukk it and stopped cause i wasn't trying to compete at a powerlifting meet and i wasn't trying to get injured either. Not everyone has the same goals, not everyone is trying to increase their one rep max and again, I'm not sure it translates to a 1rep max like that for everyone.
When i did 225 5X5, from what i remember, the 5th rep was close to failure on each set (took 60 to 90 seconds rests in between sets). So already there, I don't think i could have done 1 set of 8 reps. That's where we disagree cause your logic is if you can do x that means you can do y which means you can do z and z is close to a. That's a lot of "ifs" and i don't think it's that simple of a calculation with all the variables. Ultimately that's too many translations and it's not straightforward math like you seem to imply (in my humble opinion)
Edit: also, one thing I learned lifting weights and it applies to a lot of things beyond exercise is that the best way to get good at something is to replicate that exact thing. In other words, to get go at one rep max, ultimately you gotta go to lower rep ranges, triples and even singles. Doing higher rep range will translate some. But ultimately, doing 5x5 will make you better at 5x5, doing 1x8 will get you good at doing 1x8, and so on. It won't translate perfectly with the same proportion for everyone.