I understand how tactic works... In fact, I thought exactly the same thing as what you wrote here before actually using the excel to make some calculations.
It turns out cataphract mixing with cavalry (2:1) actually decreases the skirmish offense intensity from 1.325 to 1.122, if you are fighting on a terrain suitable for HA. This is because increasing the LC ratio increases the chance to trigger harass. Mixing with 2:3 ratio can make it to a roughly same value 1.338, however this will decrease the melee offense intensity from 2.189 to 1.397 (German brave general). If you mix too many LC, it basically becomes a standard cavalry. It's just not worth it. Of course, if you are fighting on a terrain not suitable for HA, mixing it with cavalry will greatly increase the skirmish offense, since you are simply adding number of LC from zero, but the cost of melee power is the same as previous case.
Similarly, I was arguing with someone on reddit about the power of pure camels after he posted this link. I always thought pure camels can't be better than Tie-Futu on equal numbers, because Harass is 300% and disorganized Harass is 100%. It turns out I was wrong, pure camels do better damage than any other units except mixed camels in Skirmish phase, and there is no risk of triggering volley. And at the same time they do same damage as housecarls in melee, so it's also no problem if the enemy numbers are too big to finish the battle in skirmish. It's completely viable, and somewhat anti-intuitive, that's why I suggest to include it.
I really appreciate the author because the excel sheet is really helpful to make quick calculations and get results which are not that intuitive.
I see now that when it comes to average results, pure Cataphract is better than I thought. That said, I would still use the 2:1 mix for the sake of reliability. With a mix you have a 100% chance that at least half your skirmish troops will be useful, though the chance that both will be useful is only 43%. With pure Catphract you have an 89% chance all your skirmish troops will be useful and an 11% chance all your skirmish troops will be useless. Personally I am risk averse and dislike gambling, so I'd take the one that never fails horribly. Especially since diferences in average efficiency are quite small.
I think the truth is a little weirder than this, as I've ranted about here. If it's tl;dr for anyone, the upshot is that War Elephants don't trigger heavy cavalry tactics because the line in the special troops file reads "base_type = heavy_cavalry" instead of "base_type = knights". I don't know whether "heavy_cavalry" is defined elsewhere for other reasons, but the heavy cavalry unit is called "knights" in the combat tactics file.
That's very interesting. I have failed to notice that bug concerning elephants and I quite like the heffelups and woozles solution to the HA bug.