You are making a good point here. The term "no alt accounts are allowed to be enrolled in this campaign" is indeed confusing because in fact, enrolling an alt account is allowed. It should get re-phrased to "enrolling multiple accounts in this campaign is not allowed".

For example, I'm in MixTum campaign right now but it would be possible for me to enroll an alt account (let's call it 2miau) in a different campaign, let's take CryptoGames.
In opposite, it wouldn't be possible for me to enroll my alt account 2miau in MixTum campaign because my main account 1miau is already enrolled. I would be participating with 2 accounts in MixTum campaign, which would not be allowed.

Like you've pointed out correctly, the term is very confusing and should be changed to "enrolling multiple accounts in this campaign is not allowed". Some campaigns have already implemented the clearer term.