The Manta isn't just a fighter or mere ork bomma. It is an orbital lander that CARRIES an ENTIRE ARMY. Tau Battlesuits and supporting infantry AND TANKS.
It has a metric crapton of firepower for suppressing an LZ BEFORE any of them get out and finish off the survivors. Orks or not.
In the fluff, Tau don't generally field them without escorts (which can easily take out anything up to TITANS).
Apart from that, if you come up against a tau opponent with one, he can just put it on the table and just LITERALLY crush your orks into so much plastic shrapnel. It's THAT big and THAT heavy.
Hey! Where'd ... Everybody ... go?