I'm not sure I'd agree, though it depends on exactly what one's issues are. It wouldn't be hard to go through the entrance gate and reach the carts. Going through the Siq is much the same experience on foot or otherwise. Then the Siq opens out to the "Treasury," an experience that's almost worth the trip by itself. If one stays in or near that plaza, perhaps on a a bench by the café, mobility shouldn't be a major issue.

For more on this, Google something like "Petra Jordan mobility" (without the quotes). Don't leave out "Jordan." If you do, most of your hits will be from mobility-assistance companies that happen to have "Petra" in their names. Bottom line: you won't get everywhere, forget about climbing 800 steps to the "monastery," but it's still worth it.

(I didn't see any posts on Petra in FT's Disability Travel forum, but you might want to start a thread there too.)