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(He, Him)
You may want to go back and re-read the text. It's under The Role of Dice at the start of Chapter 8 in the DMG.Rolling with it: Ability Checks for every action that has an uncertain outcome and a meaningful consequence for failure. The DM is making this hard for the players with inserting a lot of meaningful consequences. Only the very easiest actions are auto-success, only the most impossible actions are auto-fail.
To save you some time, there it reads "When a character attempts a task, the DM calls for a check and and picks a DC." That does not differentiate between PCs and NPCs.
It continues that "...using this style, you can't rely on the characters succeeding or failing on any one check to move the action in a specific direction."
Concluding that "A drawback of this approach is that roleplaying can diminish if players feel their die rolls, rather than their decisions and characterisations, always determine success."
Then also reflect on what happens if guidelines are rules?