I think that's a typo
Everything else I'm seeing says 10
Could be, though a possibility is that 15 was chosen to be more palatable from the handgun side. Back in CA, I saw a lot of, "This doesn't affect me and my guns, so what do I care?" stuff.
Maybe they think this is a way to separate people who use handguns* for defense vs rifles. We know a good percentage of the new gun owners over the last two years have leaned left. I'd be curious to know what percentage of them bought handguns and/or shotguns vs rifles (including AR pistols and the like).
*I realize most full sized handguns these days are 17-21 rounds, but the legislators might think losing a couple of rounds isn't as big a deal as losing 20 or more rounds of capacity. Nevermind still having to buy new and expensive magazines.