We (Asian people) eat Jelly fish a lot. They taste great. I think anemones would be about the same. I thin the preparation likely detoxify the venom. Like most venom, which are proteins, the get denatured from change in pH, great change in salinity, heat and other means. I don't think they all going to get stung in the mouth, esophagus, stomach and bowel when they eat these animals.
FWIW, I read that snake venom does not harm a person who swallow it. The venom get denature and digest in the stomach with our stomach acid. Although the source of this information is dubious, since it promotes sucking venom out of a snake bite, I would not want to try to find out this is true or not.