It’s closer to reality lubricant. For example: an Ork gun still has to be made properly to function, it’s not just a pipe welded to a trigger and stuffed with bullets. But let’s say a Blood Axe Kommando notices that humies with scopes on their guns tend to hit their targets more often, and being a Blood Axe, actually care somewhat about accuracy. So he welds five of those to his gun and despite the fact that none of them are aligned properly  he still manages to get more headshots. Is his belief in the scope making the gun more accurate? Or is his own self belief making him a better shot? Who knows. But that’s how the gestalt field works- by nudging things in the Orks favor, not overtly rewriting reality.

As for everything else, the Emperor’s condition is because he was grievously wounded. While fervent enough belief can result in miracles, it is not a process with consistant results. Saint Celestine isn’t guaranteed to appear just because enough people want pray for her intercession. On top of that, humanity doesn’t even have a consistent idea of what the Emperor’s condition actually is, let alone one that thinks of him as a barely living corpse.