Full Idea
Later in 'Metaphysics' Aristotle sees form as offering better prospects of separability and being a this, and treats separability and being a 'this' as better indicators of substancehood than not being a predicable.
Gist of Idea
Aristotle's form improves on being non-predicable as a way to identify a 'this'
Source
report of Aristotle (Metaphysics [c.324 BCE], hylom) by David Wiggins - Substance 4.11.5
Book Reference
'Philosophy: a Guide Through the Subject', ed/tr. Grayling,A.C. [OUP 1995], p.238
A Reaction
'Form' will be the word 'eidos', which is also Plato's word for his 'Forms'. I'm thinking that form will bestow individual identity, as in the snubness of a particular nose, where merely being 'a nose' only gives general identity.