The reason I find that believable is that their core business is selling a git server with bells and whistles. From Microsoft's perspective, Github doesn't need to be doing any marketing because they kind of are the marketing.
I would be really, really surprised if Github were the only Bay Area unicorn that lacked a product manager nagging them for more analytics. The fact that they don't need to sell the analytics is irrelevant.
I do believe Github is legitimately trying not to use that data for analytics. But whether some PM in there is querying that data for analytics purposes: at that point we're just speculating based on how cynical you or I want to be. I don't think that's a meaningful point.
Also: I'm not saying I don't think they do analytics. I'm saying I don't think they are using users' personal data for analytics. That's an important difference with respect to GDPR.