why would mark play opening act for a (great) band that only sold 35million albums?
Because he cancelled a show on this date and is trying to find another gig to fill that slot.
But he would do so as a headliner not an opening act.
If this was a regular tour date that MK's management had loads of time to work out, I agree that Mark is a more likely headliner than Toto. I'm assuming this is all he could find that works into the schedule, makes money, etc. on such short notice. It's not like Toto is just gonna give up the headline spot of a show they already had scheduled.
