But if you start consistently using a wheelchair when there is no physical reason for you to use one, will your muscles not atrophy, thereby making you need it?

I don’t think this metaphor is inherently ableist. That wheelchairs aren’t being pushed onto anyone isn’t really relevant, nor is the fact that very few people fake needing a wheelchair. I don’t think the person you replied to was shaming anyone for “faking it.” Just saying that if you don’t need a wheelchair, it’s probably a bad idea to use one.