No, a Disney Cruise Line bus transfer does not guarantee an early boarding time.
When you complete online check in on your own, you might be assigned a later Port Arrival Time and boarding group, such as a 12:30 arrival with boarding group 15. Booking Disney transportation from a hotel or the airport often results in an earlier Port Arrival Time and a lower boarding group number because Disney controls when those guests arrive at the terminal.
That said, it is not a guarantee. Disney buses do not always run exactly on schedule, and when there are large numbers of guests, you may not get on the first bus assigned to your pickup location. Some guests report arriving at the port close to or even after their originally assigned Port Arrival Time, which can result in boarding later than expected.
In general, Disney bus transfers tend to get you to the terminal earlier than driving yourself or using a rideshare, but there are enough variables that it should not be relied on as a sure way to board early. If having an early boarding time is critical to you, completing cruise online check in as early as possible and having your documents ready is still the most important factor.
