Which is a valid point. But consider this: if time gets skipped occasionally during the FMV. I.e. we didn`t get to view what happened to Squall after the flower field scene and he somehow appeared at the end = so we assume that time gets skipped. There are other things that get skipped as well.
So, how much time did it take from the flower field scene to the Seifer`s scene in reality? Approximately 1 minute? 1 day? 1 week? 1 month?
And how much time did it take from the previous scenes to the party scene?
My point is that if a friend of yours died for something he loved doing, then perhaps ... let`s say 1 month later since his death, you would be feeling better, not exactly sombre but better. It`s not like the co party went partying after the day of Ultimecia`s defeat, right?
If I recall correctly, Seifer was injured not too long before the entrance of Ultimecia`s, so how long did he get better? Who took him to a doctor? That must have taken some time.