From wdwinfo .com
“The operational hours of the cruise terminal and parking garage in Port Canaveral begin at 10:30 am. Because the terminal doesn't open until 10:30, transportation companies are not allowed to drop guests off any earlier.“
Are those hours current? (website wasn’t dated - but did not appear to be from Covid restrictions era).
I would accept this as reasonable, allowing time for departing passengers to vacate the deck. (They probably are gone by 9:30.). If they are not open, you will probably just end up in a line of cars at the payment gate.