Without claiming that X is secure: I think it’s more accurate to say that X has never cared about application isolation at all, since it wasn’t (like its contemporaries) built with that paradigm in mind.

In other words: while not originally UNIX-y, X matured in an ecosystem that idiomatically treats every userspace program as a perfect proxy for the user agent themselves. Sandboxing is a foreign (but good!) idiom that’s being bolted onto that paradigm.