I've seen that specification several times and the steel composition is clearly as you say. There must have been an addendum or waiver. The specification itself is probably a little outdated at the time of publication, anyway

. One factor might have been that the unique hardness test may have been difficult to pass with plain carbon steel, where the maker would have had to deal with inconsistent hardening and the variability of carbon allowed from batch to batch. I believe the alloys are through-hardening and the amount of carbon has somewhat less effect on the result.
I agree that we stress way to much about the steel used in an axe. 1045 on is usable, but there are small gains with better steels (carbon or alloy) assuming they are treated properly for the application.