[btown]

True - if the open source project you've contributed significantly to is, say, a well-acclaimed machine learning library, and you're applying to a machine learning Ph.D., then the sum of your accepted pull requests might be a reasonable proxy for a recommendation letter from the (presumably well-acclaimed) maintainers of said library.

[agilebyte]

It does not seems so to me. To show that your interests align, you do one of the challenges that Jeff lists in part 2. Part 1 is only to show you are a strong programmer/ambitious so your projects can be in anything it seems.