The accelerometer in the INS does measure a deceleration, relative to the reference point on the ground
As has been tirelessly said an aircraft turning from downwind to intowind, or the reverse, does not experience any acceleration in the fore/aft/longitudinal axis. An aircraft can fly around the world once in the cruise and theoretically not experience any acceleration in the fore/aft/longitudinal axis. That's why the system includes a gyro to measure angular rate. The output from the INS is combination of information from acceleration, as measured by the aircraft, and the angular rate of transport.
The start point is not a frame of reference, INS works by using a global or body frame of reference. What I think you're trying to say is the INS tracks the position and orientation of the aircraft relative to a known starting point, orientation and velocity. The known starting point may not be the point of departure, but an update from a positive fix during flight to remove accumulated errors.