I think you forget that you have to completely set aside human assumptions. How will the aliens even know they are looking at signals encoding video and audio?

I think you forget that you have to completely set aside human assumptions. How will the aliens even know they are looking at signals encoding video and audio? Do aliens have technology and knowhow in the RF regime? Will they even be able to receive, store, or analyze the TV waveforms? Do they have the notion of amplitude and frequency modulation? Also, how will they know that the signal encodes video? Absent ground truth, low entropy (or equivalent measures like bigrams, Markov models, etc.) is the only way one can determine that a sequence is a language. Even if they are able to convert the RF signal to some low-entropy sequence of symbols, there is a very real possibility of them going off on a tangent: mapping the information to a completely different language. A real world example is the symbols of Indus valley civilization. Several decades into the information age, and about a century since the seals were discovered, researchers haven't even conclusively established that the symbols encode a language.