Of course, when we say Oni (鬼) in Gensokyo, we specifically mean the Onis of "Four Heavenly Kings", which is Suika and Yuugi so far. Kasen is 99% confirmed to be the third, we just need to hear it from her own mouth. I was surprised at the question "there are two Onis in Gensokyo, now, the 3rd was just revealed" because I was reminded of Chapter 16 of Wild and Horned Hermit, where it is revealed that Kasen has an oni artifact, and Suika talks to herself that Kasen doesn't have to keep hiding, and I thought the staffs caught on to this and made a claim based on this chapter. Well, the answer, so far as the staffs said it, was incorrect. *disappoint*
"Amanojaku aren't oni, but just a particularly twisted type of youkai. " As mentioned previously by Alcoraiden, ZUN only identifies "Oni" as a species with ridiculous amount of strength. Amanojaku isn't "Oni" to ZUN because Kijin Seija, representing the Amanojaku race, are 'weak', something that is constantly a motif of the characters on DDC. At least in ZUN's definition, and in Gensokyo's definition, and also what should be in our definition, Amanojaku should not be Oni in Gensokyo, because... well, let's just say when there is a species where "鬼" is pronounced "Jaku", same as the weakest class of Mobile Suits in GunXXm, you get what ZUN is trying to say.
If we follow the logic that Kijin Seija is an "oni" because she's amanozaku (天邪鬼) (third letter is of oni character), we can say that Reimila and Flandre is also an oni due to them being 吸血"鬼". When Suika and Remilia met during IaMP, they quarreled over that "鬼" is the title for the strong.
All in all, I would conclude that question is worthy of being asked, but the answer of Q&A panel didn't do enough justice in Touhou.
I also have something to say for the Shotoku as the answer, and the doujin album that ZUN contributed, but I'll let that pass. Because all in all, it was a great panel nevertheless.