Nope--much like the font indicator in the main toolbar, that font size is inaccurate and normalized to within the text box. I'm looking for an absolute font size across the entire presentation. Try this out: create a bullet box (via Masters slide dropdown menu) with text size set to, say, 36. Select the auto-shrink option, and then fill the box with text. Then, keep filling with text. You'll notice that the text size decreases because Keynote is auto-shrinking, but if you check the text size via Format > Font, the text size will still be reported as 36, even though it obviously appears to be smaller than its initial size.
Perhaps this is an issue with only Masters slide bullets. I notice that there does not exist the ption to auto-shrink a text box if you create one.