So if I go to my boss and say I have another job X, they say choose, and I say no thanks I'll keep both. What's going to happen? They essentially fired him, but with different paperwork. Paul is being pedantic.
Therefore it's not firing.
Words meaning things isn't being pedantic. It's that words actually have meaning and you can't just change them to mean what you want -- not if you want to be understood when you communicate.
It's no different from if your office moves to a different building 5 minutes away and your boss says you have to show up at the new address and you say no thanks I'll only go to the old address. You're not "essentially fired but with different paperwork". You're being unreasonable and choosing to quit.
Expecting to keep a full-time position at one company while also being a full-time CEO somewhere else is being unreasonable and Sam chose to quit.
Oh probably worth considering that we can describe the last position you voluntarily left as being fired - you weren't going to show up anymore, so what was going to happen? You got fired but with different paperwork.