Is calc really at-risk

Hey...
Wanted to inquire about calc being mentioned as an at-risk feature here:
http://www.w3.org/TR/css3-values/

It has wide support in all modern browsers and is often invaluable for
layouts that mix fixed and fluid percentages (where border-box sizing
doesn't help)... wrote an article here for the same:
http://blog.karenmenezes.com/2013/dec/10/why-calc-risk/

I pinged Lea Verou on her blog and her reply is below:
"I doubt calc() is really at-risk, it's probably just forgotten there.
at-risk means we don't have 2 independent implementations, but for calc()
we do: In fact, every modern browser supports it. If you're worried, send a
message to www-style and ask why it's at-risk. Probably someone will reply
that it's a mistake."

Would be glad to know more..

Thanks for your time :)

Regards,
Karen