I only tried one other non-Asus USB-C 100w charger, and it still got the message 'unsupported charger'...
.. but that charger appeared to be faulty itself, with other problems (like charging starting and then stopping and then starting and then stopping, etc...).
So when I bought another one, I just went for the Asus one. Not much price difference in there, and I know for sure it's OK.
But.. Asus support told me that they basically do _NOT_ check for a brand or something. In theory, it doesn't has to be the official Asus charger, it just needs to be a 5V 20A USB-C PD charger... but apparently working ones are hard to find.