You are correct. The sample code at x is incorrect. The correct documentation can be found at https://oss.redislabs.com/redisbloom/Cuckoo_Commands/#cfreserve
So, the correct command would be RESERVE and then ADD:
$ CF.RESERVE newFilter 1000
$ CF.ADD newFilter foo
I will issue a pull request to update the Quickstart CF example. Thank you for pointing this out.