It's tricky because of the lack of concrete information. IMO the claim is substantiated by the fact that they turned off the highest simulation speed in an effort to increase server capacity - so clearly the server is tied into the basic simulation to some degree. How deep the ties go is hard to tell. If this was purely a DRM maneuver I highly doubt server capacity would be in any way tied to game speed.
My theory is that the game speed was reduced because at regular intervals your PC needs to check in with the server to: a) transfer resources between cities in a region, b) update leader boards, c) save your city.
The slower the game, the less often they have these hits to their servers.
That sounds to me like they're doing regular checkins (DRM-related, or stats, or autosaves, or something) whose frequency depend on the game speed. I just saw a review[1] which indicated that the game ran fastest when it couldn't talk to the servers.
