It is always a question of the ingredients of the coolant and how they react to a vacuum.

The leak shield will do its job, regardless of the coolant, but the reactions in the circuit could be problematic.

It is only logical that Aquacomputer does not take any responsibility.

technically that's not even vacuum cleaner levels of vacuum (typically around 200mBar). I don't believe it's worth checking vapour pressure charts yet :p
Should be fine but all manufacturers only guarantee their gear with their coolant because they can't lose time endurance testing with every coolant brand out there.
If you really want to dig into it you'll have to look at EK's cryofuel components, seek the chemicals vapour pressure charts and look how they behave at around 450mbar at the worst temperature possible, which would be around 60°C (the temp limit most pumps are rated at, or PETG's floppy temp)... or just ask them
But yea.. 450mbar, i wouldn't sweat it. it's a very light depression in the grand scheme of things.