That number seemed surprising to me - I actually expected the portion of mobile-only internet users would be significantly higher than that.
Turns out the 15% number means something slightly different. From that Pew Research page:
"Today, 15% of U.S. adults are “smartphone-only” internet users – meaning they own a smartphone but say they do not subscribe to a home broadband service."
So the 15% is people who use the internet exclusively via LTE/5G without paying for home broadband.
(I'm surprised that number isn't higher as well.)