* Make the right requests to one endpoint he found and retrieve company financials, number of hits to every google service, the name of every application running in every datacenter, etc.
* With the above two things, you know the location of services and every RPC endpoint on them, and all access control configs. You can take your sweet time to audit the 10's of millions of lines of code to find vulnerabilities and get to attack as an authenticated (albeit low privilege) user. A lot of stuff is open to all authenticated internal users.
* For example, you could take down any google service by quitting all the application servers at the same time by calling the right debugging RPC. You'd be caught obviously tho.