Not necessarily true. Premium subscribers are worth a ton more to content creators because they get a cut of the premium price for every premium view. And it's not insignificant compared to a non-premium viewer.

For a non-premium viewers to provide value to a sponsorship they need to either use an affiliate link and purchase, or at least look. Creators often don't get paid for viewers that don't click the link. And only a small percent of the users is going to do that. And even non-premium viewers already use ad and sponsor block.

Don't be confused - content creators would likely benefit greatly from a higher percentage of premium subscribers on their videos. They are guaranteed income, and sponsors are often a lot more volatile.