I tried to play around and it seems that :disabled selector is just not supported. Same for sp-action-button
Also even on a not disabled button when you add pointer-events: none, it works on a button itself (click is not triggered), but icon still receives the event and triggers a click (same happens on a stock disabled sp-button - click is triggered if clicked on icon)
I’ll just ping @kerrishotts here. Sometimes these restrictions just doesn’t make sense 
I have a feature request for my plugin which also requires dealing with disabled buttons, but it appears I’ll have to inform user, that it’s blocked by Adobe