The most interesting part of this is how encrypted traffic leaks enough info to be reliably analyzed. Really cool stuff.
For instance, let's say I can insert any word into an encrypted message. Let's say I insert the word "THE". I can compare the size of the two messages one with the inserted "THE", and the one without. If the size instead of increasing by three bytes increases by less than three. I can be certain the word "THE" is in the message or at least some of the characters. I can keep doing this till I can extract useful information out the message.
This is just the same thing. Video that is static compresses well, but video with lots of change does not compress as well.
So the camera basically let's you insert extra information into the encrypted data stream. Based of the changing size it leaks info. This one is quite simple. It just tells you where the camera is looking, making the scene more dynamic. However, the issue of compression and encryption applies to much more than just drones.
Granted, you could obfuscate things by varying the buffer length and padding the data stream (as suggested near the end of the article) - but at some point, a high-value surveillance target will probably just assume that a nearby drone is probably not operated by an avid bird-watcher or something like that and just blast the drone out of the sky. (Preferably using a laser or better still lots and lots of microwave RF - tends to get the surroundings less aggravated than using a shotgun...)
Or maybe sending the blocks out of order to be reconstructed according to a shared ordering key?
The fix is a constant bitrate with a sensible block cipher mode.