We are looking for developers who can get up and running within the GitLab application and code base very quickly and won't require much handholding or training. While for a large part, programming skills are transferable between programming languages, there is a very big difference between learning the syntax and scratching the surface of the standard library of a new language, and being comfortable enough with all of the standard library and common frameworks where you will actually be productive working on a large, production application like GitLab. The former can be done in days, the latter can easily take months.
A developer with a lot of experience in, say, Python and Django could potentially become equally proficient in Ruby and Rails in a few months, but we don’t have the capacity to actively train them, and we’d be taking a far larger risk than with someone who already has proven they can do Ruby and Rails well. We prefer to err on the side of caution as far as hiring is concerned, and the bar is already very high for developers who actually have Ruby/Rails experience, so I don't think it's likely that a developer without that experience will impress us enough to hire them.
The “we’ll consider people with experience in other languages” policy sounds really nice in theory, but it doesn't work so well in practice.