libfaketime is very cool, but I think it does something different. It focuses on, well, faking time. Fluxcapacitor makes blocking syscalls exit immediately, faking time is a necessary side effect.
For example the docs of libfaketime say:
LD_PRELOAD=./libfaketime.so.1 FAKETIME="+1,5y x10,0" \
/bin/bash -c 'while true; do echo $SECONDS ; sleep 1 ; done'
For each second that the endless loop sleeps, the
executed bash shell will think that 10 seconds have passed
While with fluxcapacitor the sleep will exit immediately, not after 1s:
$ time ./fluxcapacitor --libpath=. -- \
bash -c 'for i in 1 2 3 4 5 ; do sleep 1; done'