The internal energy of the body can only be changed by performing mechanical work.

No. According to the law of conservation of energy (U = Ek = Ep), we know that energy does not go anywhere and does not appear from empty space, it passes from one energy. This will help us understand why the internal energy appears not only due to mechanical work. Let’s take an example.
We have a ball, we throw it 5 meters at high speed, after a while it falls down and we notice that it has become a little warmer. Cause.
At the beginning of the throw, there was kinetic energy, it passed into potential, and from potential to kinetic, and from kinetic to internal, because according to the law of conservation of energy, energy does not go anywhere, and it does not appear from the air.