I think he means "written in ink" in the sense that something you post cannot be erased, much like how if you write something with a pen, vs with a pencil.

Same thing as HN btw. Ive known a few European citizens asking for their posts and account deleted, and was refused point blank.

Remember that. We're being used here just as much as on the FAANG platforms.

HN have a stated (though difficult to find) policy of removing content on request:

We don't delete comments outright except when the author requests it.

https://news.ycombinator.com/item?id=16698937#16728471

They're referring to social media being permanent, no take backsies. Everything you share is going to be on the internet for all time

S/he's updating the phrase "written in stone" for the internet age.