They just gave an example though of people who made up their own timezone because the official one was bad.
These systems exist for people and if no one other than programmers wants to do the internal calculus of “The sun is setting and they’re a quarter of the earths rotation Eastward, so that means they’re probably in bed” every time you want to call someone, then we shouldn’t make the standard that way.
You also get some things that become way more complicated, like “send the user a notification at the start of the normal working day”.
Right now you just look up the timezone in their profile and send it at 9:00, but without timezones, you need a “database of regional conventions for coordinating business hours”, which is just a worse way of having timezones.
Timezones exist because they have a purpose. UTC exists because having some sort of coordinated universal time is helpful and people (outside of Greenwich) don’t use it because it isn’t helpful to them, except in specific circumstances.
It’s like abolishing everything except latin1 because Unicode is a pain. Wanting to write your name in your traditional alphabet is just a habit that people can break.