You're quite right about the speeds being so variable that this wouldn't be so noticeable under normal circumstances, which is why at first thought even I would assume diagonal costs are x1.41--in fact, I did assume this until someone asked me a couple months ago and I was surprised when I looked into it
(I had forgotten about this discrepancy as it was an early consideration years ago)
One of the main arguments for keeping movement square is that it allows for resource costs to be consistent regardless of direction. You move a
lot in Cogmind, and are often switching directions to maneuver around obstacles or reposition for tactical purposes. On top of that some builds find themselves right on the edge of movement costs for a while, and might have sufficient resources to move cardinally but not diagonally... Being forced to move cardinally or wait until the next turn for a diagonal move is very fiddly and not very fun!
Longer diagonals would also mean that resource cost displays in the HUD are no longer always accurate, as they already show the cost of a "single move" (cardinal) compared to a stationary situation, and adding diagonals to that display would create an even larger and more confusing set of numbers. For planning purposes it's important to know how movement costs in terms of energy and heat are faring relative to per-turn generation and dissipation. Keeping the number of comparisons and numbers as low as possible is helpful.