Its more nuanced than you put it with physical keys.
Consider for a moment the baklava [1]. A baklava increases anonymity of an attacker (ie. burglar in our discussion), but also increases suspiciousness of victims and 3rd parties (ie. innocent bystanders, witnesses).
Physical burglary has many of these nuances, trade-offs.
A key on a front door is meant to keep an attacker out for as long as they get caught and/or identified, or become afraid they get caught and/or identified. It isn't meant to stop an attacker to enter a house. If an attacker wants to enter your house, they can just break in via a window. Various techniques exist for that, some are noisy, others are more sophisticated and aren't noisy.
Someone who's trying to enter via a window by night though is suspicious which means the attacker might be noticed and get caught. Most normal house keys [at least here in NL] are very cheap and have a lot of vulnerabilities, they have weak entropy (e.g. 6 entries with 6 options each), or are vulnerable via lock bumping (takes max 10 sec to enter and doesn't look very suspicious).
With electronic devices, its more binary, but it depends a lot on whom you're defending against. Petty criminals, technical criminals, or state actors.
> This is in contrast to passwords and pincodes, which can be stolen by eyeballs, cameras, audio recording devices, etc.
Yes, passwords can be stolen by eyeballs and cameras and Van Eck phreaking (TEMPEST). However if combined with TOTP it becomes something you have and something you know. Facial recognition is just a form of something you have, and the key cannot be changed.
They key can be put off, but that is not more secure than having TOTP on your watch (with yes/no button) and being able to disable the authentication method on your phone by pressing the power button 5 times.
Liveness is just obscurity. Its gonna be insecure at some point, and you'll be begging Apple for a newer, more secure solution at that point. Pray they still support your advice by then.
[1] Yes, thank you, balaclava (bivakmuts in my primary language). I have difficulty pronouncing and remembering the word. I'll keep it as-is for readability of the discussion, and for admitting my mistake.