Yes it's true, and also dependent on the format of your site as well.
For instance, google doesn't do php very well so a vast majority of the information on my website is simply overlooked by one of the most popular browsers out there.
You could try submitting a sitemap to Google, which won't necessarily speed up the process of getting your site listed and ranked, but it will certainly go a long in making sure google indexes the majority of your content.
https://www.google.com/webmasters/to.../en/about.html
Example:
For instance, I have a family tree script I host online for my family in a directory on my server named tufat (brainsmashr.com/tufat) written 100% in php.....if you do a Google search for tufat, you won't find it. Even if you do a search for BrainSmashR and tufat, you still won't find it.
However, I could submit that as a link to google via a sitemap and it WOULD index that link and start returning the link (NOT the content of the directory, just the link) when people do searches.