You're correct in the definition of 'resigning'. However, there are many cases (and this seems to be one) where people are pushed into resigning against their will. People will still resign, whether out of a sense of honour, or due to the alternative being a prolonged process which will end in the same outcome (being out of a job).

> at least supposedly voluntarily

This is the implication. As an example very, very few CEOs are fired but a great many are sternly asked to voluntarily resign by the board.

At higher levels of governance (both corporate and governmental) it's customary to not fire people. The way corporate entities/governments get around this is by asking for some one's resignation, with the implication that if it isn't given, the next step will be to forcibly fire that person. There are a lot of social and legal reasons for this, but it mostly boils down to the negative connotations that come with firing someone; it's a way of avoiding shame/saving face for both parties. The person 'being asked to leave' is spared the indignity of being fired, and the institution doesn't have the albatross of 'one of our key leaders fucked up so badly we had to boot them to the curb' hanging around their necks.

Please stop being obtuse. You know very well that people are 'asked to resign' instead of 'fired'.