Don't think it was to push back Salah - it was to push Wijnaldum and Arnold back who are normally ridiculously high up, and gave us better numbers in midfield and left Firmino to Fernandinho and our back 3 vs their 2 wingers.
Mané and Sarah would get behind but in theory the combination of very aggressively sweeping Ederson and a man advantage would be enough no matter how fast - especially if the midfield and forwards pressed well enough to not give players loads of time and space to pick a precise ball over the top.
I still think it worked for the most part - most of those balls down the side that were scaring everyone pushed Salah very, very wide because they were just knocking a ball into that channel, not really picking out a killer pass, and so we always got men back before Firmino and Mané could arrive, which is why they only got 2 shots off from those balls behind us, one of which being the Ederson save.
And those 2 were where we cocked up and gave possession away and they hit us quickly where there was real chance they could score.
IMO this is why Pep didn't change it all through the half, why he didn't pull Mendy back when it was apparent the pattern had emerged - if he was trying to push Salah back like you said, or he wanted him protecting Otamendi by doubling up on Salah like others have said, I think he'd have adjusted once it became apparent what the pattern of the game was.