You are right in your surmise that if there is enough DI water the pH of the mix will be that of DI water but as I stated in my earlier post you will not have nearly enough sparge water for that to happen.
As I also stated in my earlier post, DI water has very little buffering capacity. It only takes ~0.001 mEq of acid per liter to lower the pH of DI water to 6. That's so little that the carbon dioxide in the air (0.03%) is enough to do it. There is quite a bit more acid than that in mash. Yes, eventually you can wash it all away and the pH will rise but a noted here and previously if there is enough extract remaining to measure 3 °P there is enough acid remaining to keep pH < 6.