Eh? I don't necessarily disagree with your conclusion, but your argument is certainly questionable
Firstly, history is full of examples of new troop/weapon types appearing quite suddenly in a particular army. (And I would dispute that 100 years is a short time for technological and social changes, even in the Ancient world.) I would say it is simply not credible that all such big changes in armament or fighting style are a result of gradual changes over an extended period of time, changes so gradual that there is never a point at which an observer would notice a sudden complete change of fighting style or capability.
And purely from a logical perspective...let's say your argument is accepted as demonstrating they must have had companion-like cavalry 100 years before the Alexandrian start date. Now you can argue again that they could not possibly have developed such cavalry within only 100 years, so in fact they must have had them 200 years before the start of the Alexandrian list. Etc. Etc. Etc.