First, that source is unreliable, particularly for before the acquisition. (We don’t know whether the data is accurate, it’s based on GitHub’s own self reporting.)

Secondly, the scale is WAY off and makes it look far worse than it is. It makes it look like 99.5% availability is practically zero availability.

Finally, GitHub’s availability is not an all-or-nothing proposition. They report incidents on a granular level, for example, webhook firing can be impaired while Git hosting may be working fine.