This is just sabre-rattling, and there'll be plenty more of it before we are finished.
The EU can't exactly say Bye bye, thanks for all the money ... - because that gives the impression that they don't care, and they're not bothered about the UK leaving.
Of course, they are very bothered, so they have to come out with a fighting stance in order to be in a strong position for negotiating.
The art of negotiation is always the same, state your highest demand, know your lowest acceptance, and be ready to meet somewhere in the middle.