The technique as it isn't new of course, but I doubt this specific implementation (checking for module-supporting JS) is standard practice for a decade.
It is kind of useful. I'm trained to feel dubious of anything newly invented if the underlying elements of the solution have been there for 2 decades (there must be some reason people didnt do it that way then). If this is existing practice then it sounds right.
It’s not novel but consider how unproductive your attitude is. Someone was excited about learning something new, and you took a massive dump all over it, with no benefit whatsoever. If you’re upset that not everyone knows everything, you should be working to cultivate that excitement!