What you've picked up on is a mistake on our end where we had listed the incorrect optimal moveset for Omastar on Defense.
Omastar earns his position based on a now-legacy moveset of Rock Throw + Rock Slide. He would be a tier lower without that moveset.
His double weakness to grass is a problem, but a problem on par with the problems of other Tier 2 Pokemon.
Simulation results with Rock Throw + Rock Slide show favorable matchups vs Dragonite, Lapras, and many other common attackers.
The optimal moveset has been adjusted.
