Congrats on booking Poly, it is a wonderful resort! I have booked accessible rooms before and roll-in shower is definitely a bookable category on the regular resort side in 2019 (UNLESS they are all being taken out of inventory for a refurb in 2020, but I highly doubt Disney would knock-out all the accessible rooms at once).
This is bookable right now online for 2019:
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Were you booking through the regular reservation line for 2020? I would call back and ask another CM to check availability. Maybe the original CM was misinformed and assumed roll-in shower meant the walk-in stall shower available in the studios? You can also try calling the Specialized Functions number (407-939-7807, option 1) for room accessibility assistance.