Shane Morris @shanemo (ex 'softie designer UX guy) gave a recent talk on Being Authentically Digital with Win 8. He stated the same thing. Also that the human brain is wired to recognise T shapes making the Windows 8 start menu layout a problem.
My guess, they didn't want it to look like an iOS screen.
I agree with your conclusion, that not looking like iOS is in itself a design goal, and one that makes sense. Just as with anything else in design, a lot of people are tired of looking at iOS rounded everything.
My guess, they didn't want it to look like an iOS screen.