Would you/they discount people who have >5 years experience working for mainstream organisations that might have "grown a software division out of necessity", but no github activity?
1. I am a good programmer.
2. I have/am X.
3. (Optional) Just-so story about how X makes you a good developer.
4. Therefore, job candidates must have/be X.
I've seen this taken to stupid lengths, where employers want someone who has done professional work with the exact toolchain in use. I don't like car analogies, but it would be like hiring a driver and saying "well, we need someone familiar with the Ford Fusion, and you only have driven a Ford Taurus."