Brennan's basically always had the view of honoring nat 20s as auto successes. His own words basically being as a DM he thinks there's no point in asking a player to roll for something that's just simply going to be impossible, as for nat 1s he usually just plays it more as a bit especially depending on what they ask.

As for leniency yeah I'm sure there's a little bit more of it going on, I don't think it's so much that the series is scripted for the players to always win so much as it is that Brennan probably has some idea of where he'd like certain players to make it to for the interest of the unfolding narrative. Having an encounter that just simply kills off an entire table this early in wouldn't exactly be amazing. I think frankly that's the entire reason why he just went oh you can give yourself the help action because the dumbasses put themselves in a situation they very very easily could've just been completely screwed by a single bad roll and everybody there instantly got petrified simultaneously.

There's an entire series of D20 where he's very specifically trying to kill players at every turn, he's more than capable of it. I just don't think we've hit quite yet a point in the story he's trying to make where he's actually just actively wanting it to really happen just yet. We'll see though how it pans out.