No he was not.Yes he was.
Initially only 3 people were designing 4E This is well documented. He was later added to the DEVELOPMENT team. He was responsible for the horrible adventure which made people hate 4E, but had nothing to do with the initial gamedesign of the system. Thats why you can find the "I believe" in the quote I posted from Mearls.
About no grid being an oversight: Well it was by decision, and 13th age is the most tactical game without grid, but it was a bit an oversight over how much tactical depth you lose without the grid. Of course this was a clear decision, but it lost with this decision a lot of what made D&D 4E good.