Sure, but if that key derivation function is protected by a "you get 10 attempts then we wipe the keys" safeguard, the effective entropy is much higher. The question shouldn't really surround the effective entropy of the PIN, but rather the systems in-place to protect bypassing safeguards in the key derivation function which render the actual entropy of the PIN irrelevant. There probably isn't
no way around that safeguard, but as more of this gets moved into trusted compute silicon the level of sophistication required to breach it goes up; and is one hardware revision or operating system update away from being made moot again.
This thread really smells like https://xkcd.com/538/. Three things you have to remember, that are far more important than any of the concerns you have:
1) The effective entropy of the current system (passwords) is "shrugs shoulders fuck it not our problem". Services can enforce password entropy requirements. They cannot effectually require users to use a unique password. They also cannot forbid users from writing the password they use in a .txt file on their desktop or post-it note or throwing it in Apple Notes (EVERYONE does this outside of our bubble. Apple Notes and Excel are the #1 and #2 password managers on the planet). A six digit pin + hardware TPM key derivation is, at best, the same thing that was guarding how most people store their passwords anyway, and in many cases far better than the current state (if a user's device has no E2EE, or if they're syncing their passwords.xlsx file with Dropbox, etc).
2) Passkeys do not and are not designed to protect against nation-state level attackers. Passwords weren't either. They also don't protect well against the "grab a hammer and beat it out of him" threat vector; you're going to give up your password, and tomorrow they'll probably have your iPhone and your passkeys will be disclosed as well. Passkeys are designed to protect against unsophisticated (and even moderately sophisticated) attackers; phishing, data breaches, etc.
3) If you want higher tiers of entropy guarding your passkeys, you can do that. 1Password, as an example, already has this [1]. They store passkeys, and encrypt those passkeys with their two-level account & master password keys. Done! If you don't like 1Password, you can roll your own, and I'm sure OSS password managers like gopass/keepass/etc will eventually add this. Passkeys/WebAuthn don't prescribe to anyone how you store the private keys; Apple will do their thing, Google will do their thing, you don't have to use them, many people will, and they'll be better off (see point 1).
[1] https://future.1password.com