> The user controls their default font size on their platform. What if they'd already increased it, and now I'm just making it bigger?

The person making the site however wanted someone to change their default font size

We have to also consider that the person in question is most likely not having a bad experience on every app and website, and that this guy just had a tiny font size on his site

Even this famously tiny fonted site is readable on mobile

Most websites regardless of whether they change the browser default font size or not, do not have my preferred font size. It is my responsibility to change my own browser settings to make me have a good experience. Browser makers have made this so easy, it even remembers the setting across visits now.