I suppose that if you start with an obvious fact without any disclaimer of that then it gives the impression that everything else will be obvious too, and so the article isn't worth reading. They could perhaps have phrased it a bit differently e.g. "you're no doubt aware that ...". Personally, I wasn't offended.

It reminds of something a little different but on a similar theme: the advice to never say "clearly, ..." in lectures, because it serves no purpose except to intimidate people that don't already know what follows. Actually, it does serve a purpose: so that people who do find it trivially obvious aren't thrown off wondering if there's something deeper there than they had realised.