Board Thread:Theories and Speculation/@comment-1872728-20151008094957/@comment-4912646-20151009230303

Nativity of a World of Trees being Hiden makes more sense if you look back at where the series was at that point. When he was originally introduced, Hashirama wasn't said to be anything special Hokage-wise (Hiruzen was called the strongest) so his Wood Release wasn't the amazing unique ability it was depicted as in Part 2. Hashirama was originally given Wood Release to complete the representation of the five Chinese elements in the Hokage battle:

Fire --> Water --> Earth --> Wood --> Metal

The whole concept of nature transformation wasn't introduced in Part 1 so it's probable that Kishimoto hadn't yet decided that "natures" could be combined by having certain kekkei genkai, just like he hadn't yet decided that Shukaku and the Nine-Tails were related as tailed beasts. Thus the second databook classified Hashirama's Wood Release technique as only Hiden and Ninjutsu, not Kekkei Genkai. The first two databooks didn't make any other mistakes when it comes to classifications, unlike the third and especially the fourth which contained countless errors.

All that considered, it's almost certain that Wood Release being a kekkei genkai was a retcon introduced in Part 2 in order to make Hashirama more special. For the record, Haku's mirror technique being "Ice Release" was also a retcon; according to the first databook, the mirrors themselves are the kekkei genkai, not just a technique that uses a kekkei genkai.