The customer isn't always right. A customer who doesn't tip, when a tip is expected for services rendered is wrong.
The drivers statement is neither rude nor berating. He could use the exact same phrase to thank a customer who tipped. We weren't there. We can't base on opinion on your interpretation of the tone of his voice. I'd see your point if the driver called you cheap. Asked if you spent all your money on souvenirs. Asked you if you thought he should be working for free. He said none of those things.
He said in a way which made you feel bad for not tipping. I'll agree with PP. Had you not realized a tip was appropriate you probably wouldn't have even remembered his comment.
Not the end of the world.
The driver isn't paid to stow strollers and luggage under the bus.
DCL drivers are paid to stow luggage under the bus. They're paid a little more then DME drivers. Disney makes a point of saying tips are included for DME guests but doesn't clearly spell out which tips aren't included. I'll give Disney a lot of the blame. Either pay the drivers the extra money or clearly tell guests a tip is appropriate if the driver assists you by placing luggage under the bus.
The fact that you're not allowed to stow your own luggage isn't relevant. You can't get your own food from the kitchen, go behind the bar and pour your own drink or go into a coat check room and put your coat on a hanger.
From dictionary.com
We generally tip as compensation for a service which we ask to have performed for us under circumstances where those tips are typically part of the compensation of the employee performing the service. You're not required to use DME. You're not required to bring luggage which has to go under the bus.
Words like voluntary or present are used since there is rarely a legally enforceable requirement to tip.