Harrummpphh! Bron Nelson is correct, and my statement was wrong.
I will make the following observations:
1. Up until @turn 47, centaurs usually did not count as figures when
calculating mounted speed movement delays.
2. Since @turn 47, centaurs count as figures for delays entering/exiting
cities, and on turn 50 for swamps.
3. Centaurs still do not count as figures for calculating province to
province mounted movement.
4. A noble with 10 men and 10 horses does not always have a day's delay in
province to province travel. However, this bug seems variable and most common
when it is exactly 10 men.
Next, a design comment: if a non-Beastmaster noble can manage a string of 30
or 100 horses at mounted speed, why should he be delayed by using 30 or 100
centaurs purely as beasts of burden? He can't organize them to fight, but he
can still use them as beasts of burden, and ought to be able to get them to
move as fast as dumb beasts. This design limit to combat stacks is
interfering with ordinary trade and sticking in my craw!
Finally, I am embarrassed that I got the bug backwards. I apologize to the
moderator for sending useless messages.