My two armies are lizards and daemons and from experience and gut feeling I would rather have a horde of BL than a (unbuffed) skrox unit.
The problem with mathammer is that it takes one on one battles out of context and then attempts to apply maths to it. In this case, I believe it is even more out of context given that it is 4 units against one (and there is a mention of a Slann as well). If all your Sallies are focusing on the horde what are the rest of the daemons army doing?
Also there is a good chance a herald is in there giving hatred. you have said "Now, if the horde has a herald, balancing the points would allow the addition of loads more skinks + another krox & maybe even a 4th sally. Key is to deplete the horde before it reaches combat."
This isn't exactly true, a herald would allow the addition of a sallie and 7 skinks or a krox and 12 skinks, not a sallie and a krox and loads more skinks
But the biggest flaw in this mathematical argument is Round 1:
You say each Sallie does 20 hits, half wound, two thirds get past ward, times three, cancelling everything out and giving twenty dead BL.
This would be valid if a sallie automatically hits twenty men with every shot, they dont and the maths takes no account for the artillery dice roll.
Firstly: 1/6 of the time there is a misfire, completely messing the number of hits.
Secondly, you need to clarify number of hit for EACH dice roll, and this is dependent on your distance from the horde:
for example: If a sallie is 2 inches away from the flank of a BL horder (which is 10 inches wide) then the roll of a two would lead to 20 hits however if you rolled a 10 of the dice, you would only hit the last two "columns" of bloodletters leading to 4 hits at most. and every number inbetween:
conversely if the sallie is 10 inches away, a roll of 10 will give the optimum number of hits, however a roll of 2 will give you no hits.
This completely changes the "20 dead BL" mathematical outcome and therefore effects every single further combat.
Also given that the whole battle is out of context, it is very optimised in the skinks favour, you say walk 6 inches and fire javs from two ranks so that means they must have been 17 inches or less beforehand for this to work, why did the bloodletters not charge the turn before? Who doesnt take the icon of endless war? this gives an average charge distance of 15.5 inches, good chance they might make that charge, alternatively, if the bloodletters moved the turn before, why did the daemon player leave them in range of a move and shoot from the skrox? seems like a rookie error, if they sit 19 inches away then they prevent this move and shoot by the skrox, either the skrox moves to 7-13 inches away and risks the charge or sits there at 19 inches and the BL go after the sallies. Also why has someone let 3 sallies just march up into the flank of their BL horde? what was this person doing for the rest of the previous turns?
This is why I dont like mathhammer, however the biggest problem I have with the maths is the fact the random roll of the artillery dice is not accounted for (this is no easy equation to compute either due to the relationship between distance from target and dice roll)