Here's a video that perfectly breaks it all down but the TLDW is this:

The game has difficulty modifiers that can be added (enemies get more shields, you can't heal, etc.). No one had ever beaten the game on max heat (all modifiers) mostly because of a modifier that restricts your time to complete the game. The problem isn't killing everything - it's killing everything fast enough to beat the clock. There's basically only one build in the game to get the DPS needed but you need a series of exceptionally lucky events to happen to make it possible. This run was thought impossible not because it's literally impossible but because it was unlikely for someone to put in the mind-numbing effort to grind for hundreds or even thousands of hours just to get potential runs. The crazy thing is that Angel got the insane luck needed for a run after just like an hour of serious attempts.

Ana Diaz does a pretty good job of explaining it here