Tapping is disabled on Wacom tablets that really need it
The "tap-to-click" setting applies to all touchpad devices in mutter, with a default to off. This is a problem for Wacom devices that look like touchpads but don't have any physical buttons at all (e.g. the Intuos Pro series). On those, the tablet is mostly unusable because you can't click until tapping is enabled - that then also enables it on the touchpad which may not be intended.
I'm not sure what the solution should be here. libinput enables tapping by default on those devices but for obvious reasons users may want to disable it, that's why the option is available. You could check whether
BTN_LEFT exists on the device and make some decision based on that but that doesn't solve things either. Really what we need here is some sort of "stick to the default unless otherwise specified".