Although he does sometimes seem to equate the ideas of necessary connection and power, these are quite different ideas for Hume. It is significant that when Hume writes that "the terms of efficacy, agency, power, force, energy, necessity, connexion, and productive quality, are all nearly synonimous" (T 157), the expression "necessary connection" is absent.
Well, yes, strictly speaking, but as everyone can see, both "necessity" and "connexion" are right there, and it's hard to see why they would be there at all if it weren't for their association in the phrase "necessary connexion". A better argument, I think, would have been to point out the "nearly" and go on to show (as Ward does reasonably well) that at least some of the time the two can't be strictly synonymous. Weasel words, after all, are usually pretty important when interpreting Hume; he very often leaves himself a little wiggle room. It's one of the reasons why he is simultaneously very readable and very difficult to interpret.