Plaid can say whatever it wants. Not like it’s going to get into government anytime soon (although I used to vote for them in my younger days). You could put a donkey with a red rosette for election and it would be voted in.
Where would the airline operate? Can’t see them going head to head with any established carriers? The business destinations Brussels, Zurich etc have tried and failed. Can only think of potential sun routes which would work but do we need a national airline to operate these? Don’t think so. Would love to be proved wrong.