Yes, you can install a Sensi thermostat through HomeKit without creating a Sensi account, but please note that you will not be able to log into and use the Sensi app.
You will also not receive the full benefits of a Sensi account, including smart alerts, scheduling, geofencing, and the ability to integrate with other smart home platforms such as Amazon Alexa.
Check to see if your Sensi Thermostat is compatible with HomeKit and other Home Smart Platforms.