It is entirely necessary for the former pond space not to be able to fill up with water. How you go about that will depend on a myriad of factors specific to your former pond.

You don't need to pull the liner under those circumstances but do make sure it has plenty of drain holes in it. Otherwise rain or runoff is just going to accumulate and the firepit area could potentially flood. And if no air can get to it easily it will smell.

Pond paint will work but don't know it's longevity. Liners properly installed 20+ years if to believe this resource https://www.backyardstyle.com/best-pond-liner/. Fibreglass 25++. What you don't want is the substrate moving producing cracks when it's painted as you could come back to an empty pond. Only ever used pond paint/sealers on waterfalls so if a problem occurs it can be dealt with without causing problems to the fish.

The OP was trying to destroy a pond, not create one. ie fill it in, not fill it up.