^ it’s mostly YYC’s altitude plus the fact that WS has 320 seats in their B789s. A similar distance run out of YYZ or YVR (airports at or near sea level) would go out at full payload. But for YYC, that’s a lot of weight to carry and takeoff from a high altitude airport on a 12,500km run to DXB, and even worse on a westbound 13,170km run to SYD.
http://www.gcmap.com/mapui?P=yyc-dxb%2C+yyc-vie-ist-tbz-dxb%2C+yyc-syd&MS=wls&DU=km
This being said, DXB is doable. Tailwind going there, and the detour only adds about ~1 hour of flying, so it’s only a slight payload hit, if that. In zero wind, I believe the guy was saying a 3.5t payload hit. That's more than workable. SYD, on the other hand, is out of the question.