Where I get stuck on the whole reparenting thing is understanding that human development is quite literally a developmental process that occurs during the whole period of infancy through to adulthood. It's kind of restricted to it's time and place, iykwim? There are no "do-overs". The developmental window has opened and shuttered, been and gone. What wasn't done right in real time was done wrong, and the wrong is done.
I don't see how it is even possible *at all* for one adult to enter another adult's sphere and "re-parent" them. There is no developing child to parent. There is a fully grown - but damaged - adult. You can't "reparent" that.
That said, I do believe in healing through the therapeutic relationship. Therapists can and do cultivate secure attachments with clients, and it may well be the first secure attachment some clients have ever had (self included). Those secure attachments can definitely be healing for those that have never had one.

Maybe I am getting caught up in semantics here... maybe it is the secure attachment that people are labeling as "reparenting"?