I ran 5700 levers with a 9sp Ultegra derailleur with no issues, so I'm not surprised the 10 > 11 works fine as well. For a project I'm considering, if I was sticking with Shimano levers, I would have defaulted to 11sp levers and keeping the same 1999 Ultegra derailleurs without even thinking about it.
(not that that the latter comment tells you anything
)