First, I don't know whether the copper is actually in contact with the coolant in the liqtech, and plenty of people will tell you that with the right anti-corrosive additives, you'll be "fine". But, I keep my computers for 7-year stretches, and I'm skeptical over that timeframe if the copper is in contact with the coolant, and if it's not, there are likely thermal expansion issues.
For electric wiring, you should be told to not connect aluminum wiring to copper contacts (and vice versa) because the thermal expansion of the two metals is different, and the wiring will eventually work itself loose, spark, and cause fires. However, for cooling loops, the issue is different and even easier to understand: two different metals + ion carrier (like water) == battery (galvanic cell, hence galvanic corrosion). One metal will essentially dissolve, and really, I don't want my aluminum radiator dissolving, that would make the motherboard under it unhappy.
Here's more reading if you'd like:
https://www.ekwb.com/blog/aluminium-vs-copper/
random trivia: aluminum was once more precious than gold, aluminum oxide can be quite pretty (think rubies and sapphires) and spaceX lost their first Falcon 1 due to a corroded aluminum nut (it was replaced with a steel one -- steel + aluminum == unhappiness, and the rocket was exposed to sea spray in Kwajalein).