Slitherine

the rules say that there is not enough space to pass through cavalry retire instead.

is this just as black and white as the rules say or is their an amendment of reference elsewhere other than the passing through section?

my current understanding -

there is no adjustment of pushing enemy units if there is not entirely enough space to place the unit entirely of the other side with it's back base edge touching the square it does not pass through.

correct?

That's how we play it.

Careful players angle their cavalry assaults (as much as permitted in the rules) with a view to their post-combat follow up options.

having friends close to the your squares can block the infernal pass through move

pugsville wrote: is this just as black and white as the rules say

Thankfully YES. Just as the rules say.