Didn’t the Supreme Court rule that, unless a candidate was engaged in open bribery, campaign contributions constitute free speech?
The core of the CU decision is that engaging in political speech is not a campaign contribution. Even if you spend money to engage in that speech. Even if you pay some 3rd party organization to engage in that speech on your behalf, unless that 3rd party organization is operating in collusion with the actual campaign.
Or to put it another way, if you run off a bunch of flyers supporting Kamala Harris and pass them out, that's not a campaign contribution despite ink and paper (and your labor) not being free. If Staples agrees to print those flyers free of charge for you, Staples is not making a campaign contribution. Unless the campaign itself is involved with the process. Now, just scale that up to massive corps and political nonprofits.
People try to describe it as "deciding money is speech and corporations are people", but both of those are long held by law - corporations have had 1A rights for a long, long time and likewise arguments that restricting things used to engage in protected expression is in fact restricting protected expression have held for a long, long time (for example you can't just place a $10,000,000/week tax on printing presses to silence newspapers).