I found an instance of ‘sakkāya’ in an early Prakrit Jain text (the Isibhasiyaim), which says specifically ‘telokka-sakkayaṃ.’ The Pāli commentaries gloss ‘sakkāya’ as ‘tebhūmakavaṭṭaṁ,’ which is a similarity there.
I think Bhante Sujato’s essay here gives sufficient reason to connect the term to non-Vedic religious traditions, both because of the word ‘kāya’ being used in DN 2 for some of their doctrines, as well as Jain ‘astikāya’ which is etymologically near identical.
But it would be good to consider some of these cases.