According to https://en.wiktionary.org/wiki/machina#Declension the case of "machina" only can be nominative, vocative or ablative.
As the meaning of the phrase is "god descended on the stage by means of a machine", obviously isn't vocative and I find the ablative possible... even reasonable.
Maybe interesting link: https://www.reddit.com/r/latin/comments/366c46/deus_ex_machina_is_this_god_from_the_machine_or/