You're totally right, in part I confused >60W with >=60W, and as you rightly point out, non-marked cables should support 20V/3A.

Re: your edit. There is this exact kind of profile incompatibility between the Leviton T5636 and the HomePod mini when two devices are plugged in. The outlet should support 30W for each receptacle, which is more than enough to provide the 18W/20W the HomePod mini requires. But for some PD profile incompatibility reason (9V/3A probably), it does not get enough power.

And it is in fact terrible product design. Leviton is pointing the finger at Apple but in reality, who knows.