[crznp]

It is strange that they would claim that "non" is not a prefix but a Latin word with the same meaning as the prefix that is stuck on the front of the word.

It can still be a seperate word, but Latin "persona non grata" has the same basic meaning in English. The Latin "non plus" doesn't mean perplexed or unconcerned (e: or unimpressed), so it would be even more confusing to write it that way.