It says "mostly" public data. AMD have access to sell through figures for both, after all they supply both MS and Sony.
AMD is talking sold through to end consumers.
Nearly 30 million cumulative units have been sold through by Microsoft and Sony.
We know 18.5m is PS4. Xbox One will be less than 11.5m.
Nope. AMD only tracks the goods they supply MS and Sony.
Some millions of units are still in distribution, still on the shelf
You're right, they do track the number of units they supply MS and Sony.
But in this case they are explicitely stating sell through numbers in their earnings call. It does become clear when you read the report.
AMD are not reporting sell in number, nor are they reporting production or supplied amount of SOC's. If they were the number would be much higher than 30m as you need to account for production yield as not ever SOC will be used in a device + not ever device produced will be shipped to channel straight away.
Read their report, do some research on supply chain and production. Then come back to me.