From my reading of it it's either;Actually, all this raises an uncertainty I've always had with Retreat whether Mounted or on foot.. To Retreat you make an I check and if successful make a Move (Basic) up to D6", but is it only if this fails and the fighter remains Engaged that the enemy Engaging the Retreating fighter gets to make their own I check to get a Reaction attack, or do they get to I check to see if they're quick enough to swipe a Reaction attack at the Retreating fighter who successfully moves away having passed their initial I check?
(a) If the retreating fighter passes their Initiative check they move away and Engaged enemies test to make Reaction attacks, if the retreating fighter fails nothing happoens and the action is wasted. In this case both moving away and reaction attacks are bound by the 'if it is passed' clause.
(b) If the retreating fighter passes their initiative check they move away, if they fail they don't. Engaged enemies test to make Reaction attacks regardless. In this case only the first part is bound by the 'if it is passed' clause.
There is no 'if it is failed' clause so I don't see Reaction attacks being reliant on the retreating fighter failing. Reaction attacks either only happen when Retreat is successful, or they always happen.
I am inclined toward option (a) as it fits with what happens with Broken fighters that are Engaged, where the rules mention that the enemies' Reaction attacks happen "before the Broken fighter is moved" and then afterwards actually has an 'if it is failed' clause (Broken fighter remains Engaged, can't make any actions, no mention of Reaction attack opportunity).