Stellar/instant is a poor indicator for how secure it is, ik can be stellar/instant and not secure at all. As is the (gel)case here, I don't know if this ia a Samsung only thing or if this sensor is used in different Android devices as well. Or if this is limited to this sensor only in the first place.

You can ask the same question of face/iris scans, people have and it's quite difficult to replicate, but not impossible. There's a plethora of YouTube videos about this.