Much as it pains me to do so, I'm inclined to give Mearls the benefit of the doubt here. What I think he means is that while everyone was giving their feedback on the initial packet 2 problems they were playtesting the sorcerer and warlock internally, while we were playesting the sorcerer and warlock they were playtesting the magic items, etc., not that they're playtesting later versions of the stuff we already have. It's the context-switching that would make it confusing, not working with previous versions of anything.
When it gets to the point that there are enough interlocking subsystems that you can't playtest new things in a vacuum, that's when we should start to worry.