Many legal things are evaluated lazily: the law may not specify exactly what the vehicle is, but if such need arises, there are tools, like precedents and analogy, to answer this question.

The way to think about it is like a logical evaluation shortcut:

  if not ADA_EXEMPT and IS_VEHICLE:
    DISALLOW_IN_PARK

Since wheelchairs are ADA exempt, a question of whether it's a vehicle will probably never be risen.

Using the IT analogy, it's less like C++, where each statement must pass compiler checks for the application to merely start, but more like a Python, where some illegal stuff may peacefully exist as long as it's never invoked.

EDIT: grammar