The melting of silicon was a common phenomenon already during the second breakdown of power transistors, e.g. in audio amplifiers or in TV sets, more than a half of century ago.

In semiconductors there is a positive feedback between temperature and the current that passes through them, so once a certain threshold is passed, the current and the temperature grow very quickly until the semiconductor is melted. This is opposite to the behavior of metals, where resistance grows with temperature, tending to limit the current that passes through the metal, when it overheats.

The currents through the transistors of SOTA logic gates are very small, but their volumes are also very small, so the power density is similar to that in high power transistors.

Thus thermal breakdown that leads to silicon melting is easily achievable. This is why any modern CPU has on-die temperature sensors, so that temperature is monitored and power dissipation is limited, to ensure that the threshold that triggers positive thermal feedback is never reached.