I had a look at the video and in my opinion it is a confusing way to discuss this. An isolated proton cannot decay into a neutron in its center of mass system .
The mass of the proton is $938.27Mev/c^2$ and the mass of the neutron is $939.56Mev/c^2$ so the diagram violates energy conservation for isolated protons. It is the reason protons do not decay through such simple quark W boson diagrams.
The terms alpha decay and beta decay were defined a long time before the particle world had been discovered, they were called "radiation" and given labels to separate them. Alpha, we now know, is the decay of a nucleus to a helium nucleus. Beta was the decay to a proton with a corresponding creation of an electron to conserve charge, or a neutron and a positron to conserve charge, as in the diagram. The difference is that energy is conserved, given up by the nucleus to make up the difference between proton and neutron masses.
from the wiki link:
$β^+$ decay can only happen inside nuclei when the daughter nucleus has a greater binding energy (and therefore a lower total energy) than the mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron, and a neutrino.
The corresponding diagrams for alpha decays do not need to consider the quark composition, as the helium nucleus can be modeled as a whole.