Once they leave a moving object they start slowing down in relative position to the moviing city,
No, they don't. They're in space. The city may be moving too fast relative to the galaxy as a whole for the stargate to work, but that doesn't change the fact that as far as a jumper (or anything else) inside Atlantis is concerned, the city is standing still.
What you're suggesting is akin to saying that if you jumped up, you'd fly off into space because the earth is rotating away at 1000 miles an hour.
It's all moot, anyway, since they actually did launch jumpers in that episode to clear the asteroid field, and they had no trouble keeping up with or overtaking the city, nor did they have a problem retrieving Zelenka and Shepard from the edge of the city by jumper.
Well the city was slowing down by that point, it was moving too fast to activate the gate to send a jumper out to get a ZPM. And of course Rodney created a hyperspace engine for that jumper which was never used again. To be moving too fast to use the gate meant they were really moving, although I don't know how unless hyperspace has the rules as gate travel does on the shows, in that whatever your speed is going in it's same coming out of hyperspace.