Coulson in an undercover role as an ATCU consultant told officials of many nations of the Inhumans
OK, true enough, but that's a very recent development this season, and one the writers of the movie couldn't really have accounted for even if it would have made sense to mention it so specifically. (And I'm still not sure it would,
quite yet. At the least, it's certainly still pretty early to have the term widely known or used among the public, although I can believe knowledge of it is spreading. The first time I can recall it being used by anyone who wouldn't ostensibly be tasked with keeping it a secret was when the Watchdogs did so in the episode that immediately followed
Civil War's release, and they did seem vocally intent on further disseminating it.)
However you would think that whatever the Maria Hill lead Avengers intelligence support unit calls itself should know as much as the US, Australian, Russian and Chinese governments did and probably would have passed it on to its field operatives. Now would Stark toss it out the Inhuman name to a Peter Parker, or just assume it as the tainted fish oil explanation makes more in story MCU sense than a radioactive spider bite.
Perhaps, albeit a major point made in CW seems to be that the Avengers are operating outside the governmental system and thus wouldn't
necessarily be privy to the secret activities of the ATCU and such. They certainly don't know about Coulson & Co., even though Maria does! And Stark in particular seems to have "tapped out" of the Avengers at the end of
Age Of Ultron, so he hasn't necessarily been even so "in the loop" as they may have been. But yes, it wouldn't be implausible that they and he
could know know the term, you're right and this is mostly just quibbling. The Tony/Peter exchange
@N-121973 proposes wouldn't have felt out of place, particularly if the webslinger had replied: "What's an Inhuman?" and Tony had just said "nevermind."
Yet I'm still not sure why there would be a significant difference (insofar as the plot of CW is concerned, anyway) between Inhumans and other powered individuals that would give cause to
expect them to be called out specifically by name. I think they're well covered by the blanket references made.