Oh I get it, yes,
They could have, yes, but the activity bar is in fact suggested as 'last resort' place to add buttons / ui elements, as it uses more space overall and requires exploration.
It's suggested to (and easy once you're familiar with the api,) add buttons into other extension's (or default) views and panels, so I would expect their command to 'open git graph' was perhaps added as a button in the view/title of git related views, such as the git view or gitlab, etc. existing views.
I may be wrong, maybe they should have: I have not tried it yet, but this was just my first thought after seeing screenshots and reading your clarification :)
I'm going to try it out and correct myself if needed!
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Félix