A number of IAA compatible generators are not showing up on the "In" list; SampleTank and Alchemy Mobile to name two. Also noticed that the list heading is titled "mono sources".
I guess that the "Mono Sources" refers to the Headset Mic entry. The heading "Inter-App Audio generators" is at the same level as the "Mono Sources" heading, I guess they are separate lists in the same menu.
From the screenshot, it doesn't look to me as IAA was mono only.