Well

The biometrics only unlock the device

Yes

and give access to the security key

This is the goal, sure, but what does this actually mean on device that's mostly governed by software?

There's a chip (like a yubikey) in the device that can hold cryptographic keys.

That's good because the key cannot (easily) be extracted from the device.

That's good as long as no one has physical access to your device.

With physical access, you hope that the device's unlock mechanism is reasonably secure. That's biometrics OR password/pin.

The 'or' is the problem. For practical reasons you don't want exactly one method hard-wired. You have a fingerprint scanner (good enough), the secure element (good enough) and lots of hard- and software in between (tricky).

I'm not against biometrics (to unlock a device) because it's convinient and much better than not locking the device at all. I'm also not against device trust (which you need if you want to store crypto keys sonewhere without separate hardware), but the convience of a single-device solution (laptop or phone) comes with a risk.

If an attacker can bypass the unlock method or trick you into unlocking or compromise the device, your secrets are at risk. Having the key stored in the secure enclave (and not in a regular file on the hard disk) prevents copying the key material, but it does not prevent using the key when the attacker has some control over the (unlocked) device.

A yubikey is more secure because it's tiny and you can carry it on your keychain. The same chip inside your laptop is more likely to fall into the hands of an attacker.