Are you trying to imply that light cavalry and skirmish units in antiquity/Middle ages did not possess better mobility than heavy knights or heavy infantry units? That is an absurd contention. Scouting cavalry in all of military history are not heavy knights for the exact fact that they lacked mobility. I don't know what context you are drawing out of that book, but it is not true. Less armor more mobility, it's not hard to understand.
"...it was almost always advantageous
on the battlefield". The last three words are the important bit of context and the only relevant aspect for this topic. There is no reason in M&B to care about what scouts, outriders or flankers did because they are without any purpose if you're not dealing with a scouting plan, route reconnaissance or security on the march. And that's fine. This is not a game -- in any iteration or mod I've played -- that claims to be serious exploration of medieval military art and science and most players would get really turned off if their party would occasionally not do the things they wanted it to do.
That's why I'm fine with the current Athletics speed. It makes skirmishers pointless, but they were already pointless. Shielded infantry are effectively immune to their attacks and can rotate their entire formation fast enough to present their shields in any case you care to engineer on the battlefield. Cavalry ride them down easily -- even if you give them OP Athletics like the Jawwal's low-tier troops. They die
en masse to archers and crossbowmen. Might as well make things fun for the player so they can get the full power-fantasy experience out of it.
Especially since Athletics is one of those skills that sucks to grind up to a decent level and the realistic end-point is you move fast while wearing heavy armor (but you're still not invulnerable) and your animation looks a little bit goofy.
A heavy foot knight cannot out maneuver a light skirmish unit, they can't. That's why armies used them.
That's the thing though. A lot of armies didn't make much use of lightly armored skirmishers on foot and those that did seem to have done so out of an inability to give a full panoply or mounts to all their men, rather than as deliberate military counterplay against heavily armored troops. Even then, lightly armored skirmishers still rapidly declined in prominence in the face of improving armor (which means when faced with a choice of
something or
nothing, leaders at the time chose
nothing) and only returned to their previous position once there were handheld weapons that could reliably penetrate said armor (firearms).
Games make mobility vs. armor a huge counterplay spectrum, but that's because games are better off being made fun and interesting, with lots of counterplay options, built with verisimilitude ("this feels real") in mind rather than realism ("this is real"). Real military history can give you a pretty boring set of options.