Microsoft and Sony start thinking about next gen the day their current gen ships. whatever Cerny was doing in 2008, MS was doing something analogous at the same time. This would be likely high level conceptual thinking. (IIRC Cerny said he did his heavy thinking about X86 vs not, in 2007)
Otherwise one would ask what the hell took Sony so long, why is PS4 coming out in 2013 and not 2011?
Processor design takes years. MS said in that video that concept work began late 2010. IDC if you don't believe words straight from the horses mouth.