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Is the Space Marine Command Squad a Retinue?

yes.

OK - first this is YMDC, no proposed rules.

second, the command squad is not a retinue.
You need a SM captain to buy one and it does not take up a force org slot, but you do not have to deploy them together, nor is your captain obliged to join them and stay with them all through the game.

Also one of the major justifications for that argument about retinues got removed in the FAQ. So no it's not. But this is the wrong spot for your question.