I can see the GRU-JNB flight available in biz on Dec. 16 on the AE.com site, but can't see the UA flight IAD-GRU available on AE.com. Strange. And I agree is looks like a biz blue saver award which is available through the UA site.
Maybe call again and ask why AE can't see the UA flight on its own as AE can book the GRU-IAD flight (but of course you need the whole IAD-GRU-JNB routing).
Ah I think rank has the answer above ^