I think it can largely be a fan narrative, but I think 'playing down to their competition' doesn't have to mean anything all that extreme for the reason you mentioned.
Maybe a player or 8 aren't laser focused on game planning for the week, maybe those players or others have a few too many drinks the night before, or just aren't focusing at being 100% in the zone for certain plays or out of the gate.
It's not necessarily the difference of playing at 50% effort vs. 100%.... it could be 98% vs 100%, and that's all that's needed to not convert a couple 3rd downs or whatever.
I think the Raiders are a great example... laser focused on beating the Chiefs, and then WTF has happened the past couple weeks? There's clearly a lack of focus due to being emotionally drained or "playing down to their competition" or whatever people want to call it. That margin of focus might not be astronomical, but their ceiling is clearly far higher than the Falcons final score indicates or going down to the last second with NYJ.
And for the Chiefs, maybe they aren't digging deep into the offensive playbook for a certain team, as to not show off things they could save for the playoffs... and I'm sure they know they wouldn't need 40+ points against the Broncos, and whether they want it to or not, it could impact their mentality heading into the game.
I think you sort of nailed the issue, actually.... players and coaches have to be at the highest level of focus, and not every player is going to be at that level for all 16+ games, just like any other job. For every Mahomes, who eats/sleeps/breathes football, you'll have players who relatively take a week off or at least plays off.