From what I can recall...
Add Clonk is actually Pascal from Nier: Automata and one of the items displayed at the Collector's house was the Portal 2 gun I suppose.
Logical explanation is that since these are modded assets that already exist in HS2, Cari just did the economical thing and repurposed said assets in his story to save time and patreon funds that he could use elsewhere (i.e. commissioning modders to make custom maps, port stuff from Daz3D to HS2, or create non-HS2 mod assets that would still be useful to the story (i.e. Nova's manga, Slide art for Caly's backstory)).
From a story/lore perspective...well, the Andromeda tapes in 0.4 mentioned that the creator/s of Alien got their idea from a dream, yes? And that one of the running theories is that Eternum is akin to or IS the Astral Plane (hence the fact why otherworldly beings and spirits and dreamers are found there), yes? Well if the latter is true, then a plausible explanation is that the assets that we see in Eternum are likely the original concepts or ideas of the stuff that we see in the real world, and that they were brought into the real world by their respective creators who dreamnt of them in a dream. So it's basically a subversion of "Ready Player One" (or, god forbid, "Fortnite") where instead of all these IP from various sources being plonked into this vast VRMMO, it's the other way around.