I think the CSD was talking nonsense. If you went to your seat, found no space and came back to CE to stow your bag then I don't think you have a problem with etiquette.
If you decided from the front that the lockers looked full so preemptively snagged space in CE on your way through to row 14 then that's a tad opportunistic in my book.
As unsolicited feedback, I'm not sure that the brand of your bag, or its contents is material to the question.