I would wager some VP at YouTube in charge of shorts has their performance evaluations tied to how many hours of shorts are watched. So that's one incentive. Another is customer retention. Make current paying users addicted to shorts, and maybe they'll be more likely to keep paying.
I think you're basically right, but the comment I replied to was saying they'll somehow get more of that specific user's money. While the shorts may improve retention in aggregate, this particular paying customer doesn't want them.
It's possible that particular user, despite not wanting the shorts, will keep paying for YouTube for longer because they enjoy shorts. It's also possible that they genuinely don't like them and are less likely to keep paying because of them. People are different. What keeps some customers engaged can turn off others.
