I don't believe Radahn ever agreed to anything with Miquella, and Miquella's admiration for him didn't come with any real understanding of Radahn as a person.
Miquella describes the player as "lord of the old order" as opposed to the age of compassion he seeks, but that perfectly describes Radahn instead.
Radahn is a fanboy of the leader of the tarnished himself, a bloodthirsty barbarian masquerding as a lord who led most of the wars that established Marika's order. The red hair he got from Radagon that he takes pride in is itself a curse from one of her genocided foes. He men continue the massacres in the castles they took.
He even tried to take Leyndell by himself, showing that he sought the Elden Throne and to continue the old order. His unwillingness to let go of his horse and still holding the stars and fate in place while nearly mindless from rot show how he's an embodiment of stagnation. As far as we know the stars part didn't have anything to do with Miquella's goals, and was likely one of the things that prevented the eclipse (I think Ranni was also involved in sabotaging that for her own reasons though).

But Miquella didn't understand any of that. All he saw from a distance was some idol of strength and nobility. The remembrance spells it out; Radahn being his consort was nothing more than Miquella's childish wish.
Above all, the fact that he didn't try to bring back the horse too shows he didn't truly know Radahn.