I think you missed the point. If I have a passkey in 1password how does it become my passkey? As in, a passkey I can freely read, redistribute, and store in platforms that are not 1password. This is a property of passwords but not of passkeys.
But in general it's a bad idea to have the passkeys just sitting around in text files so the current managers are largely designed around preventing the tech support scammer from instructing grandma to dump the passkeys and email it to them.
Other than that, which is mostly only a benefit for edge cases around partially compromised devices or servers: yeah they're not much different than random unique passwords. Except they have vendor-lock-in.
But because most managers have no UI for doing this, it's impossible to trick someone into doing it.