2 things

1. While he might have said that protoss is underpowered right now, as I general rule I ignore balance BS whining and focus on the point of the post, which was that protoss lacks mobility. Definitely, it is useless to waste time and energy whining about balance. We can, however, investigate the weaknesses of protoss and attempt to find ways to mitigate them.

2. I believe the point here was mobility, not scouting. In the late game, aside from mech (very rarely used) and brood lords (also very rare), the protoss army is considerably slower than any other. This problem was somewhat alleviated in HotS with the recall ability, but it's still inconvenient, since recalling shuts down your aggressive potential. Mutalisks will lose any fight against anything that shoots up. And yet, mutalisks are the most feared unit for protoss players, entirely because they're mobile. Aside from phoenix, which isn't particularly viable at the moment, protoss lacks any kind of effective late game harassment. I mean, yeah there are zealot warp ins, but those are dependent on having an unscouted pylon or a warp prism on the other side of the map. This is damn hard against good creep spread, or really any map awareness. Possible, and done, definitely, but damn hard.

I'm not saying protoss is underpowered, because they make up for their immobility, but I do concur that protoss is the slowest race by a pretty large margin.