Yes, an integrated smart thermostat or its remote sensors can be used instead of a Flair Puck to read the temperature in rooms with Smart Vents, and also provide occupancy data. However, every Flair system needs at least one Gateway Puck.
A remote sensor cannot be used to control a mini split, window AC, or portable.
Note: Nest does not allow access to its remote sensor data by third-party products, so their sensors cannot be used with Flair.