Yes, it is true that snake can eat its own egg sometimes. Snakes in some situations can eat the eggs laid by them to recover lost energy during reproduction. They also do this in cases where they want to prepare themselves quickly for next breeding. In most of the cases, those eggs are unfertilized or damaged. In rare cases, they may be the good one. Snakes can also do that when they do not get enough food and or they are too lazy to hunt for the same. They can swallow the whole egg as they do with their prey or break the shell and eat the yolk.
Not all snakes and always but yeah, snakes can eat their own eggs which are unfertilized or damaged. They do this in order to get the energy which is lost or gone during reproduction. So this is very rare when a mother snake eats their own eggs.