It seems reasonable, as then we can look for some symmetrization argument to justify the idea. But is is also peculiar. It means that the uu and dd combinations only happen for R type quarks.
Looking at the reference of Huerta, I note that in http://math.ucr.edu/~huerta/guts/node10.html
the previous section he takes some pains to discuss the adjoint representation of U(1) and its role in the hypercharge. A subltle point here is that U(1)-hypercharge is still chiral (as Distler likes to stress) and then it needs complex representations, while U(1) electromagnetism is not.