Trying to understand directivity of an ultrasonic sensor
You are correct with your interpretation of the plot, yes.

The "-6dB Full Angle : 100°" refers to the spread angle around 0° at which the radiation is greater that -6dB.  That is, ±50° around 0°.  You can see from the plot that the radiation line crosses the -6dB mark at 50° either side of 0°, so the full spread at > -6dB is 50*2 = 100°.