Yes, you can cook using a solar cooker indoors. But as you read in the article/post, it is not quite as effective or efficient as a solar cooker outdoors, especially with today's modern windows in many homes that block out most of the UV rays.
Some windows will block out as much as 75%+ of UV rays, so it is likely most people could not use a solar cooker indoors even with a south facing window. If you have a plain glass window you should be able to do it. Here is a great example of some wall mounted solar cookersthat allow you to cook from the "inside-out"