What does it mean that the Demon King was killed by a wheelchair?

[The True Wheelchair Protagonist] In the year 1005 of the Ankylosaurus calendar, the Demon King Amberk was pierced through the heart by the legendary hero House with a sharp sword. Having reigned as Demon King for a thousand years without a single defeat, Amberk never imagined that the first human to end his life would be a disabled person. Looking at the hero before him, who was still using his hands to turn the wheels of his wheelchair, he used the last of his strength to ask, "Why...why did you kill me, and in a wheelchair no less? Were you underestimating me?!" Our chosen hero, Mr. House, truthfully replied, "Ah? I don't know. They told me about disability benefits, so I came."