These tools never work well, usually a weak combination of scraping the html, JS tags and checking DNS entries. Siftery tried something similar with a bunch of VC raised and is equally bad. The only decent one is http://builtwith.com which also happens to be a great 1-man business.
From the website: > This is a limited profile. > To see additional products added by employees and vendors, check out Siftery's profile. Both sites use the same domain provider and the same SSL cert provider. I'd say with medium confidence they are probably 2 products of the same company.
Looks like that's corroborated by the other comments on this page, which makes it even worse.