I agree with that with regard to immune monsters, but not with resistant ones.
That sidebar contradicts what the designers have told us about 5e design. It cannot be true that a party of non-casters will have trouble with resistant monsters AND that the game is balanced with only PC abilities in mind with no magic items being assumed to be present. Unless they are really, really, REALLY bad at design and screwed it up terribly.
My experience DMing a group of weapon users was that resistant monsters were a challenge as expected when they didn't have magic weapons, and not a challenge at all when they did. Based on my personal experience with that situation, I think the Xanathar's sidebar is wrong(outside of total immunity).
Hell, that sidebar contradicts itself with the beginning talking about monsters being built without characters needing magic items, and then saying later on that they might need them.