Edit: This is really strange, in a funny kind of way. I kept saying "Omnifocus" and Siri kept transcribing it as "Omni focus" and saying there was no such list. The space between words was throwing it off.
By complete accident, I discovered that Siri thinks the word is pronounced with the stress on the second syllable, so that it rhymes with "Spartucus". Or sounds like "certificate". It's kind of hard to explain, actually. As if you were saying "om NIF a cus".
Oddly, now, as I verify the Siri behavior before I post by saying "OMniFOcus" into the iPhone "Notes" app, it's spelling "Omnifocus" properly, & interpreting my "OmNIFicus" less accurately.