covers are non-canon. covers don't count. the cover does not represent what's happening in the comic itself, particularly in this case. yes, i know what comic it's from.
hulk uses his body and strength to support the lining of the enclave in which the heroes are huddled. he is not lifting/supporting/pressing/verbing anywhere near the entire 150 billion ton mass the cover claims exists. also, he isn't the only one sharing the weight.
aside from that, it would be geometrically impossible for the hulk's relatively small area of contact to at all support the entire dome-shaped hollow they were in. more likely, if he actually attempted to lift it solo, he'd redistribute the pressures around the half-spherical area and cause a collapse.
no he didn't. i don't know if you understand geometry, but the planes of the top half of the mountain and the bottom half of the mountain weren't separated. hulk wasn't the only point of connection between, thus, he was not bracing the entire weight of the mountain. the vast majority of the "bottom half" on which the "top half" was resting was holding the majority of the weight.
just saying "he's under billions of tons of rock" doesn't translate to "he alone is supporting 150 billion tons." just because there are "two miles of rock above them" doesn't mean "he alone is supporting 150 billion tons." he's holding a lot of weight, i'll give him that much. but he isn't holding/bracing/pushing against 150 billion tons. if that much weight was pressing down directly onto the hulk, the rock wall would be shattering and grinding every hero in their to dust as it takes over the "empty" area in an attempt to relieve the pressure.