There are many signals you can use to help you judge the value of a written piece of text. In this case, it reads to me much more as the writing of someone for whom English is not their native language than of someone who doesn't know what they're talking about technically.

If you want to pre-judge all technical content coming from people who have not spent huge portions of their life living in an English speaking country as being of no value, then I'm sure that will protect you from some bad content, but you're going to be missing out on an awful lot of good stuff too.

That's like complaining book writer site have shitty JS.

"If you can't write code why should I believe that the prose you write is any better?"

That being said they are selling it and that's enough reason to complain.

Not everyone speaks English as their native language.

True, but looking at author name - James Smith - it does not seem to be the case.

Potentially, but on the other hand, James Smith is exactly the sort of name you'd choose to use in the Anglosphere if your birth name was difficult to pronounce for English speakers.

It's possible to be good at coding yet poor at grammar.