Yeah, I got that when I was playing with an ESP-8266 and Blynk years ago. This feature of the deal does produce a head slap moment once in a while.
Anyway, I am thinking the line to GPIO21 is internally connected and don’t know enough if that can be remapped to somewhere else, like say one of the Grove ports on the Expansion Board. Then again I am using Meshtastic and mebe seems like cannot change much in way of the code for such a thing.
I currently measure my experience in these matters in numbers of hours.
edit: And thanks for the link to the PDF above!