I see, thank you. When I looked it up at en.wiki it notes that it can be used with the ablative -i "When used purely as an adjective": https://en.wiktionary.org/wiki/fugienti
I thought perhaps it was being used as an adjective as in 'fleeing Cicero'
No, it's used as a regular participle here, i.e. it retains all its verbal force, referring to an action done at a particular moment (Cicero fleeing). Used as an adjective it would denote a quality of Cicero's, like Cicero having a "fleeing" character or so.
Also, Ciceroni is dative so an ablative adjective couldn't agree with it.