Flash also got used for a lot of other things and in a lot of cases it was basically to add in functionality that wasn't supported by native web standards yet. This was why entire websites were built in flash. Because it gave you the freedom to do things that you just couldn't do with regular HTML/JS.
If the point the author wants to make is that creating games in HTML/JS is more cumbersome because there isn't one authoring tool that is used by everybody and has a comprehensive toolset, maybe that's true I don't know. But I definitely prefer this reality to a reality where Flash is still being used in 2020.
When you think about all the challenges the web faced early on, with all the attempted browser lock in and stuff like Flash, we got pretty lucky that things are fully open now.
And because HTML/JS are open and not proprietary, the people whining about it are free to take a shot at creating their own authoring software for games without having to worry about getting anybody's permission or getting sued by a company that owns the rights to JavaScript.
I will say, I think the niche of browser games played by kids is now taken up by phone games from a consumer perspective. I'm not sure even if an easy way to create them existed that they'd be nearly as popular.