Is a SEPIC converter truly isolated?
Is a SEPIC,CUK, or ZETA converter truly an Isolated Converter?

All three of them are not isolated. The output shares its ground with the input.
(I think you could build a Cuk that's isolated, but never done so)

How is capacitive isolation compare to transformer isolation?

You can't call it &quot;capacitive isolation&quot; if there's a shared ground.
Also, I don't think I'd call a capacitor isolation; it's meant to pass current.

How could the Common Mode Choke alleviate any isolation concerns?

By looking at it and then deciding to build an isolated flyback, maybe?
If a lead engineer would be OK with a common mode choke after a non-isolated converter, then it's possible you don't need isolation, but common mode rejection; but: that's something that the requirements engineering would have told you instead of telling you to build an isolated converter, so this all remains a mystery.