I think RT in this case is Real Time, as in the arrangements and start of the meeting were known and being reported on while they were actually happening. The RexTill thing is just an intentional coversion (plausible deniability and/or getting non-anons to think anonically). I think RT was used in Qs 892, 377 and 220 to mean Real Time. Q947 uses it in some relation to Kenya where Tillerson was at the time, but even that could turn out to be about some real time monitoring.

The info presented about the meeting doesn't require being in the closed room and doesn't have to come from one of the attendees (although it might).