This is such a weird claim to make. Salah has scored 19 or more PL goals in every one of his 6 seasons at Liverpool, and has 10 assists in 4 of those seasons. He's also played at least 34 matches in each of those seasons.
Even ignoring his ability, simply being available that often, and sufficiently in form to command a place in a team a successful as Liverpool have been in that period, is incredible. One bad injury would likely prevent any player from surpassing Salah, even ignoring talent and mentality.
Garnacho is hugely talented, probably as much as if not more so than Salah from a pure footballing perspective, but the likelihood of him surpassing Salah's achievements is tiny, simply because of all of the things that might stand in his way. Completely aside from ability, he'll need a big dose of luck to get close to Salah. And that's not a criticism of him - as I say, he has the talent, but Salah's consistency is unbelievable.