Re: So-called "smart" spackling dries rubbery
I read the instructions for this product last evening. It is specific about how to apply it smoothly, down to giving tips to wet the trowel to work it perfectly smooth, because it states on the label: no-sand type spackle.
It's apparently not formulated to be sanded after application. It's probably not a good idea to ignore the recommended uses and the specific instructions and then complain later.
Remove it with a scra per and start over with ordinary spackle or drywall mud, or any of the myriad choices available.
Remove not the ancient landmark, which your fathers have set.