Yes, it is. Exec just replaces the process image, but it's still the same process so the OS-level locks associated with it remain the same.
It's very easy to verify that it works:
lock
(
flock -n 200 || exit 1;
echo "locked"
exec ./script
) 200>program.lock
script
sleep 100
Try running ./lock twice within then next 100 seconds. You'll only get the lock once, ergo exec doesn't release the lock.