Boiling water isn’t enough to kill a lot of hardier bacteria and fungal spores and so it’s certainly not “sterilized”, though it may be ‘sterile enough’ for your purposes. Water just can’t get hot enough and the “shells” are well insulated enough to survive for hours in those conditions.
However, they also become tougher once dehydrated and so simply placing them in a really hot stove has the same issue.
You simultaneously need more heat, pressure, time, and possibly some form of chemical attack to truly “sterilize” something.
Using a pressure cooker and a tiny amount of alcohol, ethanol, is usually enough to do the trick.
If you have it, it sounds like it’s a better option than whatever easy ethanol source you’ve got. But the trick to proper sterilization is that certain microbes are resistant to different things. The Potassium Metabisulfite sounds like it’s about as nasty, but less toxic, than bleach. However even that alone isn’t enough for sterility.
Glassware is usually fine so long as you allow pressure to build and release slowly.
For what you’re doing there are additional microorganisms which can outcompete anything you don’t kill and so “sterile enough” is probably fine, but putting this here as something to keep in mind in case things go wrong.