I'm not sure I understand the process there. There's a risk the flask could shatter instead, even if it's made of borosilicate glass.
There is some small risk of implosion, but I have never seen it happen. You use an Erlynnmeyer type thick-walled vacuum flask of course. The idea of the process is to take a flask full of boiling water and steam, then cap it. The steam will have pushed out the air, so when you plunge it into cold water the steam condenses and dramatically lowers the pressure inside it. Because of the drop in pressure the water continues to boil at lower and lower temperatures. I have a suspicion that the metal particles will retain 'additional heat' for a little while which would promote more vigorous decompression events inside the particles and lead to more fracturing.
Just a hypothesis of course.