The idiom of the hill that you're willing to die on comes from the millennia old military philosophy of defending the high ground. You need to hold that hill against the onslaught at all costs, including your life, because if you lose that hill the consequences would be much worse than your own death.
Of course, yes, saying that you have a hill that you're willing to die on means that you're also willing to kill or cause significant bodily harm to those who are attacking you on that hill. When I say that free speech is the hill that I'm willing to die on I'm not saying that I'm going to let the enemies of free speech strip me naked and nail me to a plank of wood and leave me to die on a hill so long as the hill is Areopagus. I'm saying that I'm going to hold the fort until death and I'll take as many of those who siege the fort with me as possible.
If a platform takes the position that my statement is incitement of violence and therefore violates their terms of service, the mere statement that you have a hill that you're willing to die on is incitement of violence to their minds. Really, the statement that you're willing to defend yourself against violence initiated by a political opponent would fall outside of their window of acceptable speech.
Only, that last paragraph isn't entirely correct, right? If you're a conservative or, like me -- a libertarian, you aren't allowed to say that you have a hill that you're willing to die on. If you're a leftist, you're clearly allowed to call for acts of physical violence against political opponents because I see it every day. Leftists Don't even need to use euphemisms in their calls for violence on social media.
Frank Zappa famously said that the United States is a nation of laws... badly written and randomly enforced.