There are actually two Nano models, according to their paper. Bard is based on (a fine-tuned version of) the Pro model, with an upcoming premium tier featuring the (yet unavailable) Ultra model size.
#Bard users have noticed a fixed pre-span conversation to each chat session. It would make sense to hide that away behind a dedicated (internal?) Bard Assistant API, rather than exposing the full Google Vertex AI API.
Don't know for certain, though.