Yes, or to put it another way, not defining XTHREADS means "run a bit faster by disregarding thread safety".
In theory even threaded programs can use a non-XTHREADS Xlib if they only call Xlib and X extension functions from a single thread; however, Xlib and every X extension library must agree on the value of XTHREADS, and this bug arises when they do not.
Yes, or to put it another way, not defining XTHREADS means "run a bit faster by disregarding thread safety".
In theory even threaded programs can use a non-XTHREADS Xlib if they only call Xlib and X extension functions from a single thread; however, Xlib and every X extension library must agree on the value of XTHREADS, and this bug arises when they do not.