You have misunderstood because that sentence in the Youtube review was very badly worded. There is no "Obihai directory". There is no published way for someone else to find your 9 digit OBi number from your real name or nickname or email address or Facebook handle or etc. Yes one can dial a semi-random 9 digit OBi number and ring another OBi device but that is no different from dialing a semi-random POTS number from a landline phone or cellphone. And if you are really worried about receiving calls from OBi users who have nothing better to do, you can configure your OBi to reject all calls coming in over the OBi network except ones from OBis that you have whitelisted.
There is the facility to have one's own private directory and link speed dials to specific entries. No different really from a cellphone or non-basic POTS phone.
What the speaker was possibly attempting to convey is that in addition to, or instead of, using GV or other Internet Service Telephony Providers, Obihai devices can call other OBihai devices at no charge over the Obitalk network. Provided you know the 9 digit OBi number for the callee.