This reminds me of the time I was in the ladies room (if it makes a difference, it was at a professional organization, so everyone who was in that place should have been at least, well, professional-acting and choosing to be there, not trapped there by circumstances or grumpily hating where they were). There were only two stalls in this busy bathroom and there was a short line. One stall was the (slightly) larger handicapped stall. Yes, when there are only two stalls and a line, and no visibly handicapped person present or asking to move ahead of us to use the handicapped stall, we all do use that handicapped stall, agreed?
Well, an older woman in a wheelchair rolled in. She took one look at the stall, realized it was occupied, clearly saw the line, and then growled very loudly, "I hope whoever is in that handicapped stall comes out LIMPING!" Those of us in the line were pretty appalled. Anyone would have let her go ahead of us to use that stall, but she was loudly carping at the person who was already inside when she entered the bathroom and who could have had NO idea that a person in a wheelchair was coming in, and who likely was nearly done anyway....
There was a flurry of activitiy in the stall as the person clearly heard the comment and was trying to get out quickly.
And when the woman emerged from the handicapped stall, everyone could see that she was hugely pregnant!
The pregnant lady quietly said something apologetic to the room in general and clearly was so embarrassed. I know I felt horrid for her -- she should not have been called out like that. And the woman in the wheelchair sailed into the handicapped stall still fuming and grumbling with zero apparent shame that she had spoken like that and the stall user had turned out to be this gigantically pregnant woman, who surely had great need!
So: Being handicapped doesn't mean being understanding, I guess. Yeah, the biker should have hung up his bike, but as you noted -- few folks in the car, maybe he was getting off very soon. And to his credit, he did not fuss back at the woman, and he thanked you later for your concern.