If you subscribe to descriptive linguistics at all (which you must if you believe languages can naturally change) a quick google search reveals that performant is actually very much an adjective (i.e. it is commonly used as one to the point that one would be considered obtuse to refuse to recognize it as such).
To be fair one can also find a fair number of discussions about whether performant is a word in that google search, so "performant as an adjective" is clearly a newish use of the word.
Edit: after reading your reference I can comment that journal/book editors mostly do not subscribe to descriptive linguistics and it is probably good that they do not (to not take chances with parts of language that might turn out to be fads).
Your edit is a good pont. The prescriptive/descriptive dichotomy always seems a bit odd.
If you are a linguist: i.e a social scientist who job it is to explain how languages work in this world. Then you must describe.
If you are a language teacher then your job is to prescribe. And you make a good point that journal editors have good reasons to do the same. (Especially when looking after authors from STEM fields!)
To be fair one can also find a fair number of discussions about whether performant is a word in that google search, so "performant as an adjective" is clearly a newish use of the word.
Edit: after reading your reference I can comment that journal/book editors mostly do not subscribe to descriptive linguistics and it is probably good that they do not (to not take chances with parts of language that might turn out to be fads).