Uranos is simply saying an Inf vs Inf, or Cav vs Cav....not a mix of troops, and he's right.....and here's why:
The slower Inf army will sustain an attack, get damaged and then counterattack. THEN, in the SAME turn, the slower Inf army gets to make it's actual turn (counterattack doesn't count) meaning it gets an additional attack. Therefore it gets an opportunity to deal two attacks before the faster army does, provided it survives the first attack (no lucky attacks, even attack and defense ratings for both armies, etc)
Therefore it is clear that there exists a threshold for the amount of troops the defense could have that could actually survive and deal a killing blow in one round. Obviously, the slower army must be bigger (meaning more troops), but it can be done.
I'm not gonna do the math right now....(in the style of an Uranos post, you'll have to take my word for it, lol) but later I will, if Uranos doesn't beat me to it