The ScotZEB3 guidance document lists the eligibility criteria:
As long as an operator, either on its own or as part of a consortium, operates registered local bus services it was eligible to bid for funding. In Ember's case the term "local" is a bit of a misnomer, but that could equally be said for Stagecoach or any other operator with longer interurban routes.
I don't see anything in the document that specifies that the vehicles have to be used to replace existing diesel vehicles rather than on new routes.
Where else is Ember to get its vehicles from? With Plaxton having been killed off, an operator that wants to run coaches on its registered services has no choice but to look abroad, whether that be China, continental Europe or anywhere else. There's no point in complaining that public money is going to a Chinese manufacturer when your local manufacturer has exited the market!
As for McGill's, where was the concern for Scottish jobs when it ordered *checks notes* Mercedes-Benz Citaros and *ahem* Yutongs?