Is a meeting of uniformed military officers not a clear military objective who's destruction nets a clear (and quantifiable) military advantage? This would fall under the principle of proportionality: will the number of expected active military casualties net an advantage proportional to the loss of civilian life? Russian precedent does seems to indicate that they gather intelligence in the area to confirm military objectives before performing a deep missile strike directly on top of civilians, so the strike on its own is not necessarily a violation of conduct in armed conflict.

That's the risk of walking around in military uniform during an active conflict and the reason behind why there are usually so many restrictions to wearing uniform while off-duty. The only active military personnel not considered combatants IIRC are medical and religious personnel who are required to clearly display their role as such.

Edit: Generally, going out for a meeting at a cafe in Kharkiv with other uniformed military would suggest that these are higher-ranked officials. It's far from the frontline and in a big city, so you'd expect typical soldiers to not be in uniform. We simply don't have the evidence necessary for this attack and neither side wants to reveal it (Russia, because it might reveal the identify of their intelligence assets and Ukraine, because it might reveal a weakness caused by the strike and lose the PR advantage they currently have and can use to get more Western weapons).

Edit2: turns out I was wrong about this. I still stand behind uniformed military personnel being valid military targets in a war, but Russia seems to have completely disregarded the concept of proportionality in this strike and didn't even attempt to minimize civilian casualties for a strike that achieves no real strategic advantage (and in fact is likely to boost anti-Russian sentiment among Ukrainians). What a massive blunder and what a waste of human life.