Ok, so just to clarify, checking my promotions file (which COULD have been altered I suppose), I find that Sidestep, Shock, and Cover (Anti-Throwing, Anti-Melee, and Anti-Archery) all start with a prereq of Combat I OR Drill I. None of the above have a prerequisite of the other. Any higher levels of those promos, such as Sidestep II for example, only have a prerequisite of the previous level (such as Sidestep I in said example.)
All have varying tech prereqs and there are some CCs, such as Civilians, that either don't have access or have a limit to how far they can develop levels along those promotionlines. I suspect the differing tech prereqs has created a little misperception that perhaps Shock and/or Cover may somehow require the unit to have Sidestep.
Our prerequisites as they are displayed in the pedia are no longer fully sufficient to deliver a fully honest report but that's still another project in the works - mostly in DH's court. In the meantime, I can understand that some confusion may arise.
On a strategy note, if you specialize your axes against melee, specialize your swords and maces against or in defense of cities, specialize your spears against animals (early) and mounted, specialize your early throwing units such as Javelineers against archers, and specialize your mounted against Throwing units, you'll end up with the strongest power combinations (mounted still needs to gain some generic anti-throwing which will come when I get the time to develop some tags that will apply effects directly by combat class - that's the intention here anyhow.)
However, it can also be very useful to have some units that cross-promote outside of their trump specialty, such as Anti-Melee Spears etc. These units are very good at attacking stacks as they don't really care what's in there - to an extent. For situations where even cross promoted unit types like these are trumped by a third defender option in the stack, its helpful to have highly developed generic Combat promotionline specialized units - they won't be quite as strong against their foes but they can still be rather effective and capable of at least weakening the well cross-defended stack.
Adding the Throwing class deepened that by one layer. Just one. I don't intend to have a lot more of that sort of core mechanic deepening, but rather looking to develop more dynamic on the units with multiple classes like Chariots (Wheeled AND Mounted) and such, developing further for NON-combatant classes, making sure that all major classes like Explorers are brought back into balance with promos that counter them (the absence of the ability to promote against them makes them far too powerful), and then moving into deepening the combat system as a whole which will add other strategic layers entirely.
I can empathize with those saying it's 'too complicated and confusing' as it is only to a minimum extent. I still find it all far too simplistic and basic myself. Complex for the first few games sure, but then too easy thereafter because a standard practice can be established and too heavily relied on.