Whiskey has to be made entirely from grain, not from grain and other things. Sorghum spirits are not rare. Most of what the Chinese call 'whiskey' is made from sorghum. And all sorghum spirits are made from sorghum juice, not sorghum grain.
The more I deal with this subject area, the more I understand that TTB pretty much takes what the COLA submitter says except for a few things that are 'red flag' rejections, such as the words 'unaged whiskey.' Otherwise they take the producer at its word and wait for someone to object. Which is why I've gotten into the habit of objecting. Unfortunately, TTB policy prevents them from giving me substantive feedback, but I'm convinced they do follow through on my inquiries.