RCM, or recovery mode, is a separate state than Horizon. CFW's are loaded into Horizon so treat any CFW as if you're running Horizon. Now the Switch can charge both in RCM and Horizon just fine. However, with AutoRCM installed, the Switch will charge much slower in RCM. Doing this for an extended amount of time can actually lead to falsely elevated battery levels. It will still charge normally in Horizon with AutoRCM installed which is why that recommendation you see is made. You can also charge during sleep mode as its still Horizon but with a lot less functions eating up battery. Also note that RCM looks identical to a Switch powered off which is why you should not charge it when it is powered off because you will actually be charging it in RCM which I've already explained is a bad idea.I think I'm missing something here - I'm currently preparing to go the full no-fuse-burning-upgrade-path on my current 3.0.2 switch to a recent firmware version. This will require/install AutoRCM.
"For the Switch to charge it needs to be booted into Horizon / CFW." - my understanding is that, once I boot, for example, ReiNX, the charging issue won't be there? Or will it, because I've booted it from the RCM mode? If it won't be there, why would I ever have this problem/why would one stay in RCM mode with a CFW installed?
Sorry for the noobish question, the scene is moving fast and it's hard to catch up.
tl;dr Don't charge the Switch with AutoRCM installed if its off and/or has a black screen. You can charge it in Horizon or in sleep mode.