Hopefully one of the early things you'll be focusing on is the diversity of GoMix's technology (including building accessibility in by default) and its community. There's huge potential, and a great chance to address it at a relatively early stage.
It seems like the positions listed here http://www.fogcreek.com/careers are support roles instead of product roles.
"A typical startup is built around a single product, and some theory that people will pay money for that product. This theory eventually become false, and the company goes away."
The link to towerrecordsmovie.com is suggesting that many single-product companies outlive their product's usefulness, such as Tower Records. Tower Records was a large seller of physical music media (CDs, Casettes, Vinyl) which went out of business in the mid-2000s.
The author is suggesting that since Fog Creek is not a single-product company that this does not apply to them.
A lot of the time when a product or service appears to have 'gone away' it's actually moved on to serving a different audience with different needs.
So the lesson is that once a company has "made it" they should diversify so that they aren't dependent on a single income stream.
You should totally watch the film, btw. I used to shop at the Watt Avenue store in Sacramento, and the breadth of titles they carried was amazing. Easily five times the product that a store like Sam Goody would carry, and in niches you'd never see anywhere else. If there were such a thing as Celtic Rap, Tower Records would have carried it.
As I understand it, the Dublin franchise became independent of the Tower parent company some years past – or at least they advertised themselves as being “independent”.
Sorry to see that happen, but it does.
That said, the tenses and general grammar of the whole sentence don't really hold up to the usual Joel standards and sits very strangely.