Campbell
Relaxed Intensity
Well the pamphlet includes references to the gaming table being a Safe Space, especially in the quotes. Personally I don't find that sort of language at all appropriate - some tables may be set up as Safe Spaces by the GM & players, but by default they are no more a Safe Space than any other form of social interaction between friends, acquaintances or strangers. GMing in a pub, like many London RPGers, I aim to keep the norms appropriate to southern English pub culture - except I ask for no RL political conversation during the game, since players are going to have widely varying political views. Is a pub a Safe Space? That will depend on what your triggers are, if any.
Conversely I did run a different game (high fantasy BECM Karameikos/Mystara) with a specific request to keep it Family Friendly since my 8 year old was playing his first D&D campaign, and I felt pretty annoyed with the player who thought it appropriate to bring in highly adult themes like her PC brutally murdering another PC's parents in their bedroom (after they had adopted her PC), then her plotting to abduct and do horrible things to the husband of a third PC (whose player is her RL boyfriend). I definitely felt she had violated the table social contract (at the least), but I was stuck since I always prioritise letting PCs act, then following through with the natural consequences. If there had been an X-card, well, I very much doubt I would have reached for it in the moment. I guess the best solution would have been calling a time out & discussing it once her intentions became clear.
I think good faith is always necessary. In the case of this problem player, in a different game her behaviour might not have been inappropriate but I got the impression it was the 'happy shiny' nature of the high fantasy setting that attracted her to want to enact her dark fantasies there.
The X-Card in particular is not really a technique I favor. Mostly, if someone is uncomfortable with something I find it more helpful to check in and take a breather so we can actually resolve things. I get its purpose, especially for public games. I want to be open enough that we can actually hash things out in a home game.