While I agree there isn't really one definition of "true giftedness", the poster on the other forum may have been trying to convey "gifted" as distinguished from "hothoused" or "tiger-parented".

It is my understanding that research studies on "IQ stability" inform the view that some hold about gifted identification in 3rd grade (typically 8-9 years old in US public schools).

There are other threads on this forum which question the related saying that children "even out" by third grade (an idea debunked in this explanatory article by Carol Bainbridge for About.com)

In some cases, precocious children with especially large vocabularies and stilted form of expression may be diagnosed with ASD and/or given a 2e label when testing is conducted at this age.