>Is it DnD for players to be able to jump clear over the head of the enemy he has engaged to land behind him, and as the enemy watches you helplessly (due to it being turn based and it's not his turn to move) as you stab him in the back?
Yes, DnD is also turn based. you can do both of these things in DnD. Tho the first one is probably a difficult acrobatics check that either lets you land on your face or get an opportunity attack, but its still something you absoluteley can do with the apropriate skillset.
It takes two attackers to get the flanking bonus though. So jumping over someone in a 1v1 situation wouldn't benefit you in any way, unless there was a ledge behind you and you become the shover instead of the shovee.