I think that's waaaaay overthinking the whole concept.
It's important to remember - the 'X-Card' nor any of the other affiliated ideas is not a 'rule' by which anyone must abide. At no point are RPG Police coming to your game and saying YOU MUST DO THIS OR ELSE. There's no authority. There's no punishment.
All it does is provide another method for a player (who I tend to presume is a person who I want to have a good time at my gaming table) to signal that they are uncomfortable with an aspect of the game or not enjoying a thing that is happening at the table. I don't need a card or a 'rule' to respect that: respecting that is what I consider to be part of my duty as a GM. The card and 'rule' is just a way of letting my players know that I'm serious about it, and that I'm trying to accommodate nervousness they might have about speaking up about it in the moment.
That said, I've never played with an X-Card, but before running a horror game, I do outline the level of gore I expect to be a thing that happens, and ask people a) for any up-front no go areas, and b) remind them that if anything happens that makes the game turn from fun-scary to not-enjoyable, to let me know and we'll handle it immediately. Outside of horror, it's mostly the latter as a disclaimer, but I also take into account a few of the no-go areas for my regular players that I already know about (one player does not want to experience bad things happening to their character's father (any character they play), I just know that based on years of friendship, so I make sure that doesn't happen) and just build around it. Not because it's a RULE or someone's going to come in and force me to do so, but because I try to be a decent human being who is running a game for people to enjoy. I want them to have fun. If I were playing with strangers, in addition to my usual 'it's absolutely cool if something isn't working, just let me know' spiel, I could definitely see using an X-card or something like it so that all my players have a more fun time.
And I say that as someone who doesn't actually have a lot of content limits in what I can and will play or run, when I'm ready to do it. I don't squick easily, and the few times when I've been uncomfortable with where a game has gone, it's been because I've been uncomfortable with OOC elements (i.e. this male player is clearly getting his jollies off trying to get his teenaged girl GM (me) to narrate the results of him raping an NPC) at the table/in the game, not because I have problems with the content itself. Which an X-Card aids just as easily, especially when you don't want to explain something like, "Hey, I'm actually kind of uncomfortable with this (let's say) sexual content here because I and That Player Over There just broke up and I just don't want to touch sexy stuff while they're here for a bit, EVEN IF the sexy isn't normally a problem."
More tools for communication and comfort, then the better game people are going to tend to have. And that's, ultimately, what it's about.