The device you might be using is an understatement:
This literary device refers to the practice of drawing attention to a fact that is already obvious and noticeable. Understating a fact is usually done by way of sarcasm, irony, wryness or any other form of dry humor. Understating something is akin to exaggerating its obviousness as a means of humor. (Literary-Devices.com)
In this case, not only did he come on a bike, but he is apparently a very enthusiastic cyclist.
Your question also might be considered sarcastic if your intent by it was to mock the way the person was dressed.
Irony is something different, where a statement of fact seems at odds with other facts:
That man who shot himself in the foot - he's a gun safety instructor.
There is no irony in your question (rhetorical or not) that suggests that there is a disconnect between what you are observing and how the person is dressed.