That appears to be his personal anecdotal experience and admits that figure doesn't apply to mobile.
And in my experience NAT traversal
never works in the Philippines (I assume Skype was using a relay server).
This
2008 source says we don't have quantifiable data on the success rate.
Here's another source (testimony in a patent lawsuit) that 90-95% of Facetime connections are p2p (no relay), or at least they were before the lawsuit messed things up.
http://arstechnica.com/tech-policy/2013/08/report-after-patent-loss-apple-tweaks-facetime-and-logs-500000-complaints/Not all of these are NAT traversal though, some significant number are likely just connections with no NAT at all. Either way though, there are many opportunities for P2P connections, still.
I am thinking that walled garden isn't technically applicable to the wild of the internet. Apple dictates (negotiates) the terms with the iOS carriers and I assume can ensure that direct connections between iOS devices over the carriers is possible for the apps Apple wishes to whitelist.
The 5 - 10% before the change may have been amongst the Macs running Facetime that were not on a carrier network?