I didn't say that, but it's true that you can't typically buy just the middle part of the case as a replacement item. You can buy the case back, crystal, bezel, crown, pushers all as individual parts, but in order to buy a case frame, you typically have to buy a whole new case (assembly).
Again, in Omega's terminology the "case" is the entire assembly, including all of the parts. In this instance the case description includes both the codes "ST" for stainless steel, and "AU750 MSG" which is 18k Moonshine Gold. This means the case (assembly) is stainless steel and 18k MSG as I've noted. From what I can see, the bezel is the gold part.
Edit to add - as an example here is a case purchased from Omega:
Description is:
055ST1660324 | STEEL CASE CAL. 565
As you can see, it's a complete case assembly with crystal, crown, bezel, case back, and the middle part...
Click to expand...