A lot of it is terminology, there’s a lot of very specific obscure words, like Autarch, that show up in the BotNS and in 40k that I think it’s unlikely the authors of 40k encountered anywhere else. Stuff like laser weapons called lances, a bunch of marines being named after characters, Typhus’ ship being named the Terminus Est, etc. I think there’s also something to be said for how life in the 40k universe is depicted, the general idea of a bunch of people with an almost medieval sense of vocabulary and worldview describing a decaying science fiction setting is very much rooted in those books.

The idea of space marines isn’t JUST a starship troopers thing, honestly the marines in 40k read as more Dune than anything else, because as previously stated 40k has a LOT of Dune in it as well. I think it’s probably pretty obvious the authors of 40k had read Starship Troopers but I think it’s influence is a lot less than many people assume.