ugh. I hate this. I'm not sure I have good advice, but merely wanted to commisurate. But what DH and I have taken to doing (after being quite annoyed by several parental responses to our DS3 is this: Say the minimum, and don't offer information unless specifically asked.
ex: Parent of another kid in DS's preschool seeing DS writing a sentence on a magnadoodle.
Parent: "Wow, he can write?"
Me: "Yeah...he really likes letters." (that's my go-to for the 'my kid can't do that' statement...I say he's really into math, or letters, whatever is in question.)
Parent: "When did he start writing?"
Me: "Oh, I don't know, he's been doodling stuff since around 2 or so." (understatement, but close enough).
End of conversation. (and it's even worse when they remark upon his prodigious counting skills and I seriously fight to the urge to say, "counting? What about multiplication?

)
Basically, as annoying as it is to pseudo-hide this stuff...I just got fed up with the skepticism, questions about drilling, etc etc. So we stay pretty mum about the whole thing. (that's one nice thing about this forum!)