Intended use. If you advertise your flavor as being diacetyl-free then the FDA could consider it a tobacco product. There is no other reason to advertise it that way since diacetyl is only a concern with respect to inhalation. In other words, for vaping. Now if you were to sell it to manufacturers only I'm not sure how that would play out. But selling it directly to consumers would very possibly put you right in the tobacco product manufacturer category according to the FDA.
O.... K....
I'm not sure Intended Use could be Taken that Far? But then Who Knows. Never thought the FDA would completely Blow Off the UK Health Ministries Data.
So Stranger things have Happened.