Earlier quoted context omitted.
The trouble is you get advice in a context, and that context might not be known. The context for your advice is that writing long sentences is/was popular poor style. Especially for young and new readers of English.
My observation is that in Romance languages writers (and readers, presumably!) like to create long sentences with lots of sub-clauses. It's almost a game where you have to read the whole 50 word sentence before you can parse it fully, having to push onto a mental stack quite a lot of state that you can only pop at the very very end. So in Romance languages one often finds sentences as long as entire paragraphs are in…
This is interesting to me, because I do the exact opposite. I write my clauses more or less independently, then try to string the related clauses together with linking words punctuation in a way that flows naturally.
I can't imagine composing long, complex sentences on the fly, and the only people I've seen that manage to do this in conversation are unusually erudite.