The article is a little unclear, but I think they're saying that it was compressed between pieces of diamond, then transported still pressurized within the diamond cell.

Any phase change and polymorphism costs energy if the energy required to disorganize the crystalline structure is high it should be able to be maintained under pressures it can’t form under.

Diamonds don’t break apart just because their structure requires high pressure to form.

As long as the melting point of the ice at 1 atm pressure I don’t see why it would break down but material science isn’t my forte hence the question.