Interjections may not be very representative of the phonology of a language. For instance, "shhh" (being just a consonant) obviously doesn't obey the minimal word restrictions in English. Now, functional words like "they" and "that" can behave in weird ways too and often don't need to obey quite the same restrictions as lexical words. But functional words tend to have high token frequency, so their effect on a language's sound inventory is comparatively strong. In English, this means constant reinforcement of the distinction between /θ/ and /ð/, even while the voicing distinction is often lost in lexical words. Interjections, on the other hand, probably have low frequency.
So my guess would be that interjections don't often get used in minimal pairs, or when they do get used, it's with greater care and more caveats. I suppose you could look for other clues as to whether you have a phonemic glottal stop, such as cooking up a small perceptual experiment for yourself.
I don't know if you already checked out some of the literature on characterizing marginal contrastiveness. I've come across Goldsmith 1995 (his introduction to the Handbook of Phonological Theory) or Hall 2009 (her dissertation) for a more quantitative approach.
