Browsers remember your zoom settings on a per-site basis. Try using it before claiming it will fuck up the entire experience.
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
https://support.mozilla.org/en-US/kb/font-size-and-zoom-incr...
So no, you don’t have to set the default.