a) an isolated squad, not a chapter
b) xenos pretty much suck at war. all of their tanks have armor protection that is <300 mm, which puts them on par with 1950's shit. Eldar firearms have a lower effective range than modern small arms, and their standard body armor can't protect against water-tipped bullets. Tau soldiers are trained almost exclusively in ranged combat, are assisted by targeting shit in their helmits, and still cant shoot better than a trained human soldier. . Necrons are buggy, a group is used by some backwater world's PDF as target practice due to some error. They would still be a credible threat, but i just can see modern militaries getting roflstomped by a a race that pretty much finished technology, but can still be torn apart by converted tractors.
Really chaos, orks and nids are the only ones that would outright fuck us, because of corruption, spores, and biomass harvesting respectively.