BTW isospin is actually how many up vs down quarks they are. It's not a fundamental property like spin or charge.
It's an old term that was created before they knew that up and down quarks existed.
Personally I find the term outdated because there are 4 other quarks, and isospin only talks about two of them.
Isospin symmetry: https://en.wikipedia.org/wiki/Isospin #History :
> Isospin is also known as isobaric spin or isotopic spin.
Supersymmetry: https://en.wikipedia.org/wiki/Supersymmetry
Does the observed isospin asymmetry disprove supersymmetry, if isospin symmetry is an approximate symmetry?
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.
This video just drive home that using isospin and hypercharge is like using epicycles to describe the motion of the planets: It works, but it's overly complicated, and it's better to just use the actual thing (quarks, and heliocentrism).
> using isospin and hypercharge is like using epicycles to describe the motion of the planets
No, it isn't; those are actual quantum numbers of the electroweak interaction below the symmetry breaking energy (the other such quantum number is electric charge).