Yes, you are reading correctly. The 100-400 L is slightly better than the 70-200 with extenders at 280 and 400mm, and the Tamron 150-600 and the 100-400 are within a whisker of each other. Canon TCs are overpriced.
The Canon TCs won't fit on to the Tammy so I couldn't test it. But, I would guess that the Kenko etc ones will give terrible IQ and AF.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22