The claim that there's an optimal time under tension for hypertrophy appears to have come from Charles Poliquin, who wrote in his 1997 book ("The Poliquin Principles") that 20-70 seconds of total time under tension per set is desirable for bodybuilders. No citation was given for this claim.
The idea that 8-12 reps is the optimal rep range for hypertrophy is an even older dogma that probably can't be attributed to any single author.
Both of these ideas are wrong.
Research has reliably shown than rep range does not matter for hypertrophy, as long as sets are done sufficiently close to failure and the reps are not so ridiculously high (more than 30 reps per set) that it starts turning into cardio1, 2. Sufficiently close to failure means that if you can lift 100kg for 15 reps, or 140kg for 5 reps, those two will have similar effects on hypertrophy, but lifting 100kg for 5 reps cannot be compared to lifting 140kg for 5 reps. Stronger By Science has a good write-up summarising this research.
Research has also shown that while time under tension does increase hypertrophy3, this is counteracted by the fact that you can lift less weight (or do fewer reps) with a deliberately slowed tempo than you could when lifting at a normal speed. The overall result is that lifting light weights for low reps with a slow tempo, lifting light weights for high reps with a fast tempo, and lifting heavy weights for low reps with a fast tempo all produce similar levels of hypertrophy4, again provided that you're training similarly close to failure in all cases.
Probably the only important thing to take away from this is that slow tempos or higher reps can act as a substitute for heavier weights for hypertrophy training, which may be useful to people who have limited access to equipment due to Covid-19 restrictions.