If I read it correctly, it was a support person that provided them with assurance. Not an executive or vice president or manager or vp of sales. GitHub did not give them permission nor their approval; it was a single person in support department.
Who relies on support people to determine the basis of their business when it’s obvious that they were concerned with the high usage rate and that it might cause problems for their customers?
Oh yeah, support staff aren't always aware of everything. For example, with one of our latest features, we didn’t have time to add a UI option to disable the feature. The expectation was that support staff could disable it via their special admin panel upon user request. However, I accidentally discovered that when users asked to hide the feature, tech support told them it wasn’t possible! It turned out the tech support lead forgot to share that information with the team.
As for the OP, they should have conducted load testing and implemented rate limits on their end, rather than blindly relying on someone’s word that GitHub was ready to handle all their product's load for free.
Who relies on support people to determine the basis of their business when it’s obvious that they were concerned with the high usage rate and that it might cause problems for their customers?