I don't understand how business works
MS think GitHub is worth more than $7.5bn if they control it.
GitHub owners think it is worth less.
Neither share their ideas about valuation, but both think their lot is improved by the sale.
Whether either has any sense is another matter. And who is right can only be known in the fullness of time.
I've not seen a single comment anywhere giving a calculated value, so I doubt if anyone knows.