Interesting. I've never actually heard of moves being run alphabetically. The manual claims turn resolution is simultaneous, and in the case of two armies from different nations entering the province of another nation, it indicates the first army to arrive is determined randomly rather than by alphabet order. Moreover, it indicates that it only determines which army arrives first in the case where three or more sides are present in a province at the end of the turn, and not at the time each movement order is resolved.
In large part, what the case being described by Abysia and Man relies upon is what sort of province is being targeted for the move by the non-attacking army: friendly or unfriendly. Apropos of the great big turn resolution sequence list, friendly movement is always resolved before regular movement, and all movements are resolved before movement-related battles are processed. After that comes any castle storms, random event battles, battles from magic item/monster effects (Chalice, hi-yo), and discovered sneaks, in that order, and before all of this (even friendly movement) is movement through magical rituals and consequent battles, but that's likely irrelevant.
In fact, it seemed so curious that I thought I'd run a quick test, since it's fairly trivial to set this condition up. Pure vanilla, no mods. Map is Talis, since it's teeny, with Abysia and Man the two players. Pretenders are irrelevant. It took three turns to move both armies into position over the bridge, facing each other. I ran three tests with each, which is far from statistically significant, but I'm lazy that way.
Case 1: Abysian army moves into province occupied by army from Man; Man army moves into different (independent) province.
Result 1A: Man's army moves away. Abysia takes undefended province.
Result 1B: Man's army moves away. Abysia takes undefended province.
Result 1C: Man's army moves away. Abysia takes undefended province.
Case 2: Man's army moves into province occupied by army from Abysia; Abysian army moves into different (independent) province.
Result 2A: Abysia's army moves away. Man takes undefended province.
Result 2B: Abysia's army moves away. Man takes undefended province.
Result 2C: Abysia's army moves away. Man takes undefended province.
If someone's more bored than I, they can probably play around with
the save as they wish, though I can't imagine why.
EDIT: Struck out a false statement from my own presumptions, but otherwise left it present for posterity. In fact, it doesn't actually matter; if all movement is resolved before all combats except in the case of two armies marching into each other directly, then moving into a friendly or unfriendly province is irrelevant, because it will still be processed before combat begins.