The author says that he starts with a single html file and splits it or incrementally adds stuff when needed.
That course of action has proven to be a really bad idea on every non trivial web project I worked on.
Mostly for teamwork and maintainability reasons.
E.g there is no clear project structure, the next dev will not understand stuff and do things differently. And welcome to the chaotic legacy code hell. It's like Php all over but in Js.
I usually work for customers who to some extend now what they want. I choose an appropriate tech stack for that use case. The team can use that stacks conventions to develop the project. There is no need to slowly grow, everything can be done in parallel and due to the conventions stuff sticks together in the end.
This sort of work flow has a tendency to just pile incredible amounts of code in files thousands of lines long. That is going to be unmaintainable in no time.
Everyone claims they don't do that but they all lie (yes you too).
Also everyone has a chance to end up maintaining the resulting amorphous blob (could be you again).
This lacks tooling like hot reload, spell checkers, linting and so on. You will need that anyway so why don't you start with it?
Also you need tooling for deployment like config files. Tests, Ci and what not...
Lack of documentation. Since this follows your personal style your colleague needs to follow. A framework provides documentation for that. Working like this, the docs need to be written by the devs. Although they may claim otherwise they usually don't.