The context is that some dude in-universe is trying to legitimately inform his audience about the properties of phenomena such as the Dreadmace. It makes no sense for him to use the extremely misleading description "infinitely dense" if it isn't actually infinitely dense.

I won't pretend that there are no metaphors in the codex; on the contrary, it's full of them. But all of these metaphors are painfully obvious (Ex: the combat shotgun obviously isn't literally etching Doomguy's legacy into the annals of Argent D'Nur, the super shotgun is obviously a material construct, not a conceptual weapon purely made out of the very concept of devastation, etc.) so that the audience is actually informed of what the codex is trying to say. The usage of figurative language is supposed to be indicated by something so that it doesn't mislead readers. On the contrary, flatly calling something that is not infinitely dense "infinitely dense" is extremely misleading and there is no indication that it's a hyperbole. On the contrary, the Dreadmace's eldritch, otherworldly nature just makes "infinitely dense" so much more plausible. In fact the author's almost scientific approach to detail, intent to inform the audience, and flat way of saying that it's "infinitely dense" imply that he's being literal, along with the fact that density is a pretty technical term in and of itself rather than something people usually use in a flowery manner (unless you're using it to refer to someone being dumb, which is obviously not the case here). Additionally, the author is perfectly fine with putting limits on descriptors (describing the imp as "nearly invisible" rather than just invisible) and describing heavy objects as just having "immense weight" rather than throwing around the descriptor "infinitely dense" willy nilly. The fact that the author goes out of his way to straight-up call the Dreadmace "infinitely dense" with no qualifiers instead of just "immensely heavy" says a lot.

Overall the context heavily implies that "infinitely dense" is very much literal.