Right at the beginning of the video he says, “You keep adding neutrons until you end to the point where they spontaneously decay and then you end up with something of the same mass but a different element”. As far as I know, it’s the number of Protons that determine the element so how is that possible?
The mass of a proton and neutron are almost identical, so you could have a different element with the same mass so long as the sum total of protons and neutrons in the nucleus is the same.