This line of thinking can and is used as an excuse for all sorts of security shortcomings. Yes, in threat modelling, this would be give fairly limited opportunity as an attack vector, but any successful attack could have a massive impact. It makes sense for device owners to consider this possibility in how they protect their machines, and for device manufacturers to attempt to mitigate it.

That is not what this attack does. You steal the laptop and then do a trick to unlock the secrets without giving back the laptop. Your MITM is fooling the fingerprint reader and the OS, not the user.

The point of the system is to protect against an attacker who steals the laptop so it is a complete failure.