Well, it is more complicated than that.
Prior to McCain-Feingold, it was already illegal for corporations to directly advocate for or against candidates in elections - but it was defined using “magic words” - the ads couldn’t say “support”, or “vote for” or “vote against”, etc - but they could make ads saying things like “Call Candidate X and tell them to vote against this bill…” or something like that.
The point of Section II of McCain-Feingold was to close the sham Issue advocacy loophole. As I said before, CU overturned parts of both McCain-Feingold and FECA, which was the prior existing law.
Prior to CU, there had been only one SCOTUS case that addressed the question of whether corporations had first amendment rights at all.