Ahaha, now are going pretty deep. Let's be nerdy together!
1) First of all, you brought a very right point that Disco uses expansion tank instead of overflow tank. That's why there is no radiator cap. I didn't realize that.
2) Yes, again, expansion tank cap is supposed to vent the air once it reaches to the limit. If my argument sounded like "the cooling system must be vacuum-sealed", that is my bad. It is not, it can't, and, more importantly, it shouldn't be. I just meant "properly sealed". It might explode someday if it is vacuum-sealed everywhere. The system can hold the pressure to a certain level for sure, but it needs to vent after the limit.
3) Yes, again, because the vented air contains some of evaporated coolant, we would lose some small amount of coolant. So my arguments "there won't be any evaporation in the process" and "The coolant won't go anywhere" are misleading. I want to rephrase those as "there won't be any significant evaporation in the process if the system works properly" and "The coolant won't go anywhere under normal conditions if the system works perfectly." I will explain those in the next.
4) If there is too much coolant, the expansion cap will vent the air. So we lost some of the coolant. If we repeat this process, we would lose more coolant again and again. In the end, we would reach to an 'optimal' amount of coolant naturally. In that status, the overall pressure of the system would stop at just below the maximum. It won't reach to the maximum easily now as there is less coolant to expand. This is what I meant
"The coolant won't go anywhere under normal conditions if the system works perfectly.". Now the cap won't vent anything, so the law of conservation of mass persists.
5) Even though the cooling system works perfectly, under certain conditions like long/fast drive on a very hot day, you will lose the coolant anyway. However, that is something minor you can just top off coolant on 1 year/10,000miles maintenance. Less than half inch mostly. Also, very likely you won't see any alert too unless you have faulty/sensitive sensor. Most garages will do it for free if you pay for oil change service. This is what I meant by
"there won't be any significant evaporation in the process if the system works properly."
6) However, if the cooling system doesn't work properly (i.e. can't cool the engine down effectively) with some reasons, the temperature will keep going up, expanding more, increasing pressure, and making the cap to be vented. Now even though you don't have enough coolant, it still expands too much due to overheating. So you will see the alert. You add more coolant. But you didn't address the cooling system issue. So you will lose coolant again and again. In the end, you will see the alert again very soon. This stupid repeating adding and losing coolant is not expected in the proper cooling system. This happens when your cooling system is out of order.
7) So, yeah, if the system works perfectly, it won't evaporate. And if the system works properly, it won't evaporate easily at least. That's why I mentioned that any mechanics who say "it can evaporate" are all BS. By accepting that, you will end up losing coolant unnecessarily, see the alert frequently, have to keep adding up more than once a year. It is better to find the root cause. That said, I do understand that it is a quite difficult, painful and time-consuming job to find a minor leakage. So lazy mechanics just give up and say "just top-off". But some experienced guys are able to identify the cause fairly quickly and give you a peace of mind.
Sorry for being nerdy. But I believe we all can learn something from this