There are two people in a picture. Each is riding a bike. There are two bikes, and they are riding their own bikes.
The question is:
Can they ride a bike?
The answer is:
Yes, they can ride a bike.
They can ride bikes.
Why is "ride a bike"? There are two people there. Don't know why. The answer is singular not plural. Why?
"they can ride bikes' means that each and everyone of them can ride bikes!
How dexterous of them. Most people can only ride one bike at a time.
The picture is showing that, give them a bike, any bike, they can ride it. Yes, they are showing that they know how to ride a bike.