If by "capability" you mean not supporting some latest standard that provides marginal benefit over established solutions (e.g, Wi-Fi b/n/c/whatever, "cloud printing") or anti-features (Apple's "Pro" touchbar), then yes. But if you mean being able to do anything actually useful, I'd say that maybe only those working with Audio Processing still are under-served by Linux Desktop - and even that will change soon with Pipewire.
No. The point of the discussion was about hardware support. Wacom tablets, USB video game controllers and custom editing panels (like the ones for DaVinci Resolve) are AFAIK supported on Linux just fine.
You are really grasping at straws here. We are talking about Linux support for devices in a traditional work environment and I can seriously say that has been more than adequate for more than 10 years. How many backward-incompatible API changes does Apple push every couple of OS versions?
How many dongles were people forced to buy to continue using whatever peripheral they had already working?
The position was quite clear: Linux is more than capable to work as Desktop or business computer for the most common industries and that its support for common peripheral devices has been pretty much in "just works" territory, provided these peripherals are not using or dependent on extremely new protocols or standards.