Hello,
although Alfresco is open source software, it is not a software that is developed in an open source "style", i.e. have a group of volunteers / unassociated developers. The code base is still actively developed and controlled by the entity Alfresco Ltd. and its employees. Community code contributions can be introduced via JIRA tickets, but I guess there won't be a direct community write access anytime soon - or "official" community collaboration branches.
As for Git in general or Github in particular:
I hope Alfresco considers migrating to Git in a mid- to longterm perspective, as it would greatly simplify applying minor local patches and subsequent upgrades of the local code base, without dependency on the Alfresco controlled SCM server (I don't know how much transfer they have to support per day).
I don't think Alfresco will ever allow the codebase of their main product to be hosted on an external service like GitHub they have limited control over and would be dependent upon.
As I am not an Alfresco employee, this is just a personal opinion / statement. Maybe Jeff Potts or someone from engineering can provide an official statement.
Regards
Axel