Keep in mind that they went to this guy's house because their research turned up indications that he was the season's big bad, and then they found him in a wheelchair. If they were suspicious that he might be faking, that's not at all the same thing as picking some random disabled person on the street and suspecting them of being a villain because they're disabled.
But it's still dwelling on the wheelchair as the single overriding thing of significance about him, and that is prejudice. Substitute any other characteristic for the wheelchair -- the same gender, the same skin color, the same height -- and it's obvious how nonsensical it would be to think that way. So you're singling out "being in a wheelchair" as something intrinsically Other, something so abnormal that it overrides every other aspect of a human being in your attention. And that is ableism in a nutshell. Being preoccupied with the one thing that makes someone different from you, instead of just accepting it as one of the many attributes that make up a person, is what prejudice is.