I thought the "drive strength" was the current delivered while the port is set as an output, whereas the pullup or pulldown is active only when the port is an input (?)
Perhaps you were writing this while I was still editing my comment. yes drive strength is about GPIO outputs, and pullups are about inputs. As I said my "educated guess" is a pullup strength of about 100 uA, but I could indeed be wrong by a factor ten, Gert should know more, perhaps he can provide us with a "functional diagram" of the GPIO, including pullup strengths, and more info about how "drive strength" works.
P.S. I would be very surprised, (and concerned) if the GPIO would, at any point in the boot process, really be set as outputs. I do think that what you see is indeed the turning on, and later off, of weak pullups.