There is a school of thought in tax theory that charitable donations aren't tax expenditures because they're not within the normative tax base. It's not a widely held view, but it is a view that does have some adherence, and that continues to this day.
On the specific question of the $36 million figure, I would readily agree that it's speculative, because it's based on a number of assumptions that arguably do not hold true. A realization of capital gains from a donation of capital property is very much unlike any other disposition of capital property for the very reasons you mentioned. So I assume that number derives from an assumption the donor would have sold the shares on the open market but for the donation. That may not have happened. They might have been held and sold at a time when there was a capital loss, which would radically impact that tax expenditure calculation.