Regardless of whether or not the battery charges in RCM, the concern about possibly being unable to charge the Switch was a false alarm. If your Switch dies with AutoRCM installed, the system will still trickle charge enough to be able to enter RCM and then load Horizon to charge it back to 100%. If the battery is too dead to enter RCM but not 100% dead, there is a small chance the system won't trickle charge. If that happens, the battery either needs to be disconnected and reconnected, or it needs to 100% die.
As for whether or not it charges in RCM, most of the people who know what they're talking about say it doesn't charge in RCM, and they have evidence to back it up. One user who knows what he's talking about claimed it did charge in RCM, but he didn't actually check and was merely going on the battery percentage after using RCM for extended periods of time.