The Disney website has said this for a long time, and it's misleading. WDW buses start running much earlier than that, just for the benefit of people who have early ADRs in the parks. (We're WDW veterans and have taken these early buses many a time to get to an early CP breakfast.) If you get to the bus stop between 7 and 7:15 AM, you should be able to catch a bus and be there on time with no problem. There is a turnstile entrance at MK dedicated to those with ADRs before park opening. I believe it is the turnstile farthest to the right. There will be sign designating this turnstile, so just look for that. You may or may not be asked for your ADR confirmation number, so it would help to have that with you.