No language is perfect. There is nothing wrong with making your own that works the way you want a language to work. I don't get people who complain about new languages. Variety is good. Let people have fun.
Looking at the explanation you linked, I would say that the author uses the language in a very unidiomatic way (if idiomatic Jai would be a thing :D). It takes a very template focused mindset and uses the features of the language to bastardize and complicate things needlessly (IMHO, as a not game developer). I think my main take away from the language is that it focuses on the way memory is mapped (similarly to how C does it) and less on behaviour associated with that memory.
So for that example, instead of trying to overthink a container that uses a generic behaviour through templated types, it would implement different overloaded functions for each type as needed.
It makes little sense to announce a language with a lot of details over so many years without providing a decent specification. Either the author is interested in feedback, then a specification or language report is the established vehicle among language designers, or he is not interested in feedback, which raises the question why he publishes any details at all.
> It takes a very template focused mindset and uses the features of the language
But as it seems that's a focus point of the language; why otherwise the author would take the effort to even modify structure declarations with compile time expressions.
> that it focuses on the way memory is mapped (similarly to how C does it)
What do you mean by that? And is this really a unique selling point if it is similar to C?
> Blow is not trying to "sell it" to anyone.
For that, he is doing a lot of promotion for his new language with all the youtube sessions and blog posts about it.
If it's promotion for anything I think it's rather for his sokoban like game.