Unions do pay lobbyists. But it depends of course how much money any union has available. And ECA is lobbying in brussels on behalf of the single unions as those can only lobby with their national governments. I would say that the ECA budget is way too low for the amount of work they have to do, it is a very tiny organization.
Big companies always have much deeper pockets for lobbyists, even the small ones can afford usually more than the unions can.