This does appear to be a bug in XCB, in that there is no reason why a single-threaded binary that makes use of an X extension shouldn't run against both XTHREAD and non-XTHREAD copies of Xlib without difficulty.
This does appear to be a bug in XCB, in that there is no reason why a single-threaded binary that makes use of an X extension shouldn't run against both XTHREAD and non-XTHREAD copies of Xlib without difficulty.