Fact in Fiction [closed]

I don't see any real problem with using the word fact in this context. At the very least, it is logically valid. If Deadpool and Wolverine exist, then Deadpool can heal better than Wolverine. They certainly exist in the Marvel universe.

One may sometimes talk about facts in a fictional universe using the word canon. While in some sense it means official (as opposed to derivative), I believe it can also mean factual given the confines of the agreed-upon universe.

Interesting question. Such a statement is a fact within the context of its fictional universe(s) -- I see your question as more of whether or not you should reference the context in which such a statement is fact. Since Deadpool and Wolverine are completely fictional characters that cannot be confused with reality, the fictional context is implicit within the statement. If it was a statement using real characters or things within a fictional context (was Abraham Lincoln's bear bigger than Benjamin Franklin's T-Rex?), then it would probably be appropriate to explicitly define your context.

OP is taking pedantry to extremes here. In Lit. Crit. contexts (which we can assume are usually written by people competent in English), it's perfectly normal to come across...

"Faustus is in fact..." (171 hits in Google Books)

"Othello is in fact..." (867 hits)

"Macbeth is in fact..." (751 hits)

The term "in fact" usually introduces information the reader might either not be aware of, or be inclined to disagree with - but it's information in context. The context can be a fictional "world", just as much as the "real world".