What "resignation" can mean in the context of such a situation for top high-level people might not be your typical resignation concept for e.g. employees, or even middle management.
For example ministers of most gov "resign" when they are told to do so, and yes they do resign, even when this is not from their own initiative, and even if they absolutely would not have done it if not told to. For other structures, you can have various kind of pressure playing a role.
We will probably never know the exact degree of pressure and the comprehensive list of who did it, the alliances, maybe the threats etc, and technically it absolutely was a resignation, but given RMS further actions in the GNU Project (that is: not resigning despite additional attempts to leverage his departure from the FSF to make him do likewise here too), I can only say I have my doubt on the total amount of practical freedom he had concerning the FSF part; and it is now probably not just him phoning back and saying: "oh by the way I changed my mind I want to come back", but more likely other "political" and/or alliances changes at the top letting that be even possible.