But i think, they are not fully wrong. They are having a war right now, so those systems will be used against Russian Aviation.
In Greek case, in order to be targetted by S-400 systems, Greece needs to attack Turkey with Aviation so that Turkish S-400s would target them. Turkey being on the defensive.
What if Turkey attacks and iniates a war, Greece responds with retaliotary strikes and gets targetted by S-400. So, Greece would be targetted while fighting a defensive war.
So, Greece also have a point. But with this logic, nobody should sell anything to both Greece and Turkey.