It would seem I messsed up there, yes. The problem is, I can't do this because I have a modchipped Mariko Switch. Or rather, I can't take the risk. I don't understand the why and how behind it, but I was explicitely warned to NEVER press POWER and the +/- volume buttons lest I want to brick my Switch (apparantly it works differently when you use a modchip instead of a softmod).
Update: I tried exosphere out, but as I feared, it's too late already. The nag is in the system and I'm stuck because as far as I know the Mariko Switch - for whom the rules seem to be different from the regular softmodded Switch almost everyone uses - can't enter maintenance mode via POWER + volume buttons.