[wging]

The sixth footnote appears to address this a bit. You of course need to have something that could possibly grow before you can hope to achieve growth.

During Y Combinator we measure growth rate per week, partly because there is so little time before Demo Day, and partly because startups early on need frequent feedback from their users to tweak what they're doing. [6]

...

[6] This is, obviously, only for startups that have already launched or can launch during YC. A startup building a new database will probably not do that. On the other hand, launching something small and then using growth rate as evolutionary pressure is such a valuable technique that any company that could start this way probably should.

In other words, if a startup hasn't launched yet they quite sensibly don't measure growth rate--but it's a good idea to launch early so you can measure growth and optimize for it.

[dave1619]

Another option is if you need time to build your product, you can launch another "something"... like the legendary mvp video from Dropbox or the finance blog from Mint.com they used to draw in signups several months before launching.