Perhaps. The way I read it, the choice of "proper written sentence" gives it the flavour of "Proper. Written. Sentence" - as in, with an annoyed emphasis. As

Perhaps. The way I read it, the choice of "proper written sentence" gives it the flavour of "Proper. Written. Sentence" - as in, with an annoyed emphasis. As always, you can apply rules to English if you like, but sometimes they are optional.