Coworkers are trying it and it's not working -- the google response says there was a problem with it, and not much else.
I do not have a yubikey (at least not yet 😄), and I'm really not good at the GUI stuff so I don't know as much about it as I probably should. Searching the fora here found a question and comment that didn't make a lot of sense to me, but again I admit to a lot of ignorance here.
So any pointers to where I should be look would be appreciated.
Passkeys and security keys are supported in WKWebView, if the RPID is for an Associated Domain using the webcredentials association. In this case that means only Google-owned apps will be able to authenticate to a Google URL. If you need a web view to authenticate to a domain you don't own, you should use ASWebAuthenticationSession.