I agree with your point. But 75-300IS's contrast and colour is pretty good as well. Its in the "better" lens range. Alot of time, people just think L lenses are supposed to be "better" but I have come across consumer lenses that out perform some L lenses. 2 good examples are 75-300 IS and 24-85mm. These are very sharp and contrasty consumer lenses, especially 75-300 IS.
Also, he uses a canon flash which have ETTL, so ETTL compensate according to camera settings, so it fine.
To jump from f4-5.6 to f4, not much difference under low light, but with IS, at least camera-shake is minimise. I'll recommend a f2.8 for low-light to minimise mothion blur in low-light, rather than a f4.
Just my point-of-view, I had a 70-200 f4 for about 2 months, sold my 75-300IS for it for the same reason. I felt its not very "helpful", like 75-300IS under low-like. In the end, I found a 2nd-hand 70-200 f2.8 at TCW at a pretty good price. And its been with me eversince. The difference between the f2.8 and the f4 is significantly obvious.