[dudu24]

This misses the point of isospin. Isospin is an approximate SU(2) symmetry due to the fact that the up and down quarks (the "light" quarks) have very similar masses compared to the rest of the quarks, so they can be approximated as two different eigenstates of the same particle. It's mathematically identical to the SU(2) symmetry of a spin-half particle. The reason it doesn't include the other quarks is because they are so much more massive.