"Battery shop says cells all check ok, no stand-out bad cells - uniform voltage cell-to-cell. Sulfated from lack of use - but not as bad as they often see. It has flat plates - easier to de-sulfate?"

technically they may check ok as far as voltage stability and equal range across cells which is a good thing. The part that is not accurate is about what they said about 'sulfated from lack of use'. That's not when the main creation of sulfation occurs, sulfation is mainly created during the charge/discharge process. And one other thing is when water is added to cells over time it's the type of water used, using regular tap water is the worst because it is full of minerals which is what is the main contributor of sulfation. This is why they tell you to use 'distilled water', this water is processed and most of the minerals are filtered out which helps reduce the sulfation effect.
So they are not telling you the whole story and are just telling you the minimal reason about sulfation in the cells.
Most of the sulfation is in the bottom of the cell because it breaks loose from the cell plate and settles to the bottom of the cell.
Now as far as straight plates, that is good if they are not wavy, heat can cause the plates to warp and if that happens the cell is damaged and cannot be guaranteed to be stable or last any length of time. So straight plates is a good sign. And no, it would have no profound effect on being easier or not to de-sulfate. Also a de-sulfating charge only breaks away the sulfates from the plates and that settles on the bottom of the cell. And the effect will like brewski says will depend on how sulfated the plates are. If they say they are not as bad then the cell may be good for a while longer but like i've said its not what you can see, its the part you cant see (in the bottom).
Remember, every cell plate is designed for a predetermined number of cycles, anything you get beyond that is just a blessing but you are operating on borrowed time.
I would make them explain how the 'reconditioned warranty' they offer works (if they offer one) and how long it is for. Chances are they will only warranty the new cells they install (if any were installed), they probably won't warranty any of the old cells.