Mixed dominance game theory

I don't quite understand here, how exactly is the strategy $$(\sigma_1(U),\sigma(M),\sigma_1(D))=(0,1/2,1/2)$$ dominates $$U$$ since if you look at the first column, $$5$$ is greates than $$3/2+4/2=1.5+2=3.5$$ . I'm guessing i'm doing something wrong. Could someone please tell me what i'm doing/thinking wrong?