As a fan of the Chargers, I have seen this excuse made on this forum several times about the difference in offenses in an effort to explain the difference between the performances for Tagovailoa and Herbert. For a different perspective, please consider that the Chargers have very arguably the worst OL in the entire NFL (worst per PFF, 31st and 32nd in pass and run block win rates, respectively, per ESPN). I actually read in another thread that one or more posters said the Charger OL was above average, which is absurd. And FWIW, Rivers has had a Hall of Fame caliber career, so the fact that Herbert had a better season than Rivers had the year before is telling.
As for skill position players, I agree that when healthy, the Chargers had better players in 2020. However, Keenan Allen started only 13 games. Mike Williams started only 11. Austin Ekeler only played in 10 games and in a couple after he came back from injury, he was clearly not 100%. Meanwhile, Herbert had 3 TD passes each to such household names as Jalen Guyton, Tyron Johnson and Donald Parham and another 2 TD passes to Gabe Nabers--all undrafted free agent nobodies until they teamed with Herbert. Herbert was PFF's top rated QB under pressure and accomplished everything he did despite being saddled with absolutely awful game plans and play calling. With even average play calling and in game strategy, Herbert probably would have put up another 10 or so TD passes and many more yards.
And everyone forgets that he did what he did in only 15 games as Anthony Lynn decided that it would be best for Herbert not to play in 2020 (just to give slightest glimpse into the sheer ineptitude of the coaching, coaching that would have completely negated the best rookie QB performance in NFL history, but for a medical accident).
I hope Tagovailoa works out for you guys and you all should be stoked about your upcoming draft, but I think it is okay to admit that Herbert is significantly better than Tagovailoa at this point. Maybe things will even out over time, maybe not. Ultimately, we will all see.