OK. Since it's clearly visible it's obvious that they are lying to some extent. But since the whole point with IR is that we can't see it, it's impossible to tell how far into the IR spectra the red light on the flashes go, without a different instrument than just the eye.
Anyway, since the AF optics contains an IR blocking filter, it seems reasonable to believe that you can't focus with light that's IR only.
My thoughts are that the reason they use the red LED focus assist on a flash like the 580 EX II are not exactly related to IR or not, but instead for a couple of other reasons:
- The bursts of "real" flash light other units use are rather annoying, and also consume a bit of the charge level from the capacitor.
- Red LEDs were the first type that could be designed, and are still about the most efficient.
- The optics in front of the LEDs make them project a pattern which allows focusing even on surfaces where there is no contrast to detect to begin with.
The last advantage you can try against a white wall in the evening. With the lamps lit, you can't focus on a wall that has no lines or edges. But turn off the light, to make the AF assist in a 580 EX II turn on, and you get a pattern the camera will focus on, with any AF point you prefer.