> it occurred to me that they probably are hosting this site with github but are using their own domain name
That is exactly what they are doing, and in itself this is not at all a problem.
But presumably the link worked for the original poster, so either there is a dynamic DNS problem (we are being sent to an address that serves the .github.com certificate an not the "right" one but he was sent somewhere that does have a certificate for that name) or* someone is resigning content and his machine is set to trust their CA certificate. This latter cause could be normal/expected (his company having a MiTM policy for regulatory monitoring reasons) or his machine could be cracked by an external entity.
That is exactly what they are doing, and in itself this is not at all a problem.
But presumably the link worked for the original poster, so either there is a dynamic DNS problem (we are being sent to an address that serves the .github.com certificate an not the "right" one but he was sent somewhere that does have a certificate for that name) or* someone is resigning content and his machine is set to trust their CA certificate. This latter cause could be normal/expected (his company having a MiTM policy for regulatory monitoring reasons) or his machine could be cracked by an external entity.