Q. - Is the 2" rigid pipe (mast riser) with a threaded male pipe end attached to a bypass meter socket in a 2" meter socket hub (female pipe thread) a weather tight connection?

I would say yes... what is the rest of this story? Has someone claimed that it is not? If so, why not?

I also say yes, the socket hub connection is labeled as "3R" - but and inspector is insisting it needs a weather tight 2" nipple with a rubber grommet and nut - which would mean the male nipple end is attached the socket hub - the same scenario as before, plus now you have an additional joint. I think some inspectors do not want to admit when they are wrong about their conclusions (code enforcement).

You need to show the inspector the installation instructions for the hub and meter socket, both of which should identify acceptability for exterior installation. If it is labeled "3R", you win...
NEC 110.3(B) Listing and Labeling... this code section is your best friend or worst enemy...
If you can't show the inspector that what your doing and the equipment your using is "listed and labeled", you will always lose the argument.

Any and all threaded rigid conduit connections are weathertight.
The inspector is an idiot.