The article stated you would be working in the area you wish to work in, so if you are a developer you will most likely be working on WordPress or something relating to it (I may be wrong, just a presumption).

Considering that Wordpress is open source and any candidate applying at Automattic as a developer will most likely be familiar or know of it, the chance of having a developer not being familiar with it in one way or another are slim, even more so if it's someone actually wanting the job.

If I was looking to be hired by a company working with open source, I would most definitely do my homework up fornt and familiarize my self with their "flagship product" as it were.