The problem you are talking about is a common problem of Fischer. All pots and pans are like this, except that they are not designed to be "vacuum-proof" (it seems to be a very simple thing, but it is not made, and the brand is too big). So is Zyryl. Pots and pans are difficult to wash. It's really a big project to clean the pot after cooking it once.

The pot can't be opened because the pressure difference between inside and outside increases after the air inside is cold. The method is to heat the pot until it can be boiled.