<blockquote>... but am struggling to figure out how this could have been an innocent mistake.</blockquote> What's stated in your equipment quote? Was there any direct discussion of a two-stage thermostat? Any discussion or mention by either you or the installer of keeping the original thermostat? A single-stage thermostat sometimes is used with a two-stage system for "reasons" ... such as keeping the job to a price-point or avoiding the need to run additional thermostat wiring if that presents some challenge. The system will work although not to the best design-performance.
The T4 line does have a model that's 1C-2H for one-stage/speed heat pumps with one-stage of auxiliary/backup heat, but your stated compressor model 4A7A7036A1000 is a cool-only unit ... maybe the dealer's installer team overlooked that? Do you have gas for heating?