this is what I understood when I studied such stuff at school
In other words

Mass and energy are the two sides of the same coin

The point is that it does not seem to be the modern nterpretation of the equation

Take matter annihilation
A proton and an anti proton turn into a radiation of pure energy
The photons of the radiation have "momentum " but they dont have " mass "
Im many books, even at university level, you can read that the mass of photons at rest is null while their dynamic mass is given by the momentum of the radiation divided by c
Anyway strictly speaking such claim is false