Actually, it is kind of wrong. The monster math doesn't assume that you have +X weapons. However, it IS assumed that you'll end up with several magical items by level 20.LOL! You are not wrong, I was referring to mid-hi levels, where I think most martials will have +1 weapons ... but of course I can't prove that. But certainly has been true in all D&D eds. beforehand.
Also, there is a thread here on EN, someone did the stats on the 5eMM, the preponderance of Magic Res was very large at mid-hi CRs. Will have to track down thread to get you #s ...