You are wrong read the errata ...they follow the rules for normal cav and monstrous beasts
Mate you need to read the errata more closely
Neither the book, nor the errata entry say that the mounts of a monstrous cavalry unit can attack from subsequent ranks, and neither the BRB or the errata refer you to the rules for monstrous beasts at any point.
"Monstrous Suport" is not a generic rule that functions the same in all cases, it is worded differently for monstrous infantry, monstrous beasts and monstrous cavalry and should be read separately for each, you can't just copy a section of rules text from the monstrous beast rules and paste it into the monstrous cavalry rules just because you want to.
Sorry, but the FAQ and BRB do not support your interpretation.
From the BRB p. 83
"all the cavalry rules apply to monstrous cavalry with one exception-monstrous cavalry always use the highest wounds characteristic the model has..."
Now read the rules for cavalry p. 82
"When a cavalry model makes supporting attacks, only the rider can strike - we assume that the mount lacks both the discipline and the reach to fit through those ahead of it"