I saw some discussion about how in (some versions of) the myth, Tamamo-no-Mae was purified and became good upon touching the stone, not trapped inside it.
Then, they started speculating that maybe her evil and malice were split from her, and they became its own evil kami, and that's what's trapped inside the rock.
The funny part is that it was clear they were making this up on the spot, not constructing it from historical myths. When given the opportunity to tell a story about an evil spirit sealed away a thousand years ago in a magic rock that kills you if you touch it, who has now escaped, which we know because the rock split in half in real life, and who is now presumably causing chaos across the countryside, people aren't going to let minor technicalities get in the way. (I wouldn't let them get in the way, either.)
Then, they started speculating that maybe her evil and malice were split from her, and they became its own evil kami, and that's what's trapped inside the rock.
The funny part is that it was clear they were making this up on the spot, not constructing it from historical myths. When given the opportunity to tell a story about an evil spirit sealed away a thousand years ago in a magic rock that kills you if you touch it, who has now escaped, which we know because the rock split in half in real life, and who is now presumably causing chaos across the countryside, people aren't going to let minor technicalities get in the way. (I wouldn't let them get in the way, either.)