Yeah there are two types of people who like animals. There are the people who are passionate about animal welfare and become vets, biologists, trainers, park rangers. There are people who spend a lifetime getting to know animals inside and out and do everything they can to protect them and raise awareness about actual environmental issues.
Then, there are people like this, who make placards and protest what the other lot are doing.
They only make one claim in their article, that because they do 'demeaning tricks' they are healthy enough to be released. Yeah, fit for release has to do with their survival prospects, not how fast they can swim. It has to do with life skills and natural instinct. Most of the animals at Sea World were captive born, they have no experience with the wild. Those that weren't born at Sea World are either rescues (and hot tip, if they think they'll survive in the wild they release them. That's a parks an wildlife decision too not a Sea World decision) and those that were wild born are now so old that they're past the life expectancy for wild Dolphins.
It's worth noting that not all the dolphins at Sea World are used in shows, in fact the vast majority are not. Dolphins also when they get older, they do 'retire' from shows and get to live a leisurely life. Also worth noting is that they arent asked to work harder than wild dolphins who have to hunt their food. If Sea World had dolphins and didn't get them to exercise in feeding sessions THAT would be cruel.
Finally, doing things for an audience isn't cruel because there is an audience. You ask your dog to sit at dinner time before you feed them? It wouldn't be cruel if people payed to watch your dog sit when you asked it now would it? There's nothing inherently wrong with getting an animal being asked to perform behaviours for a crowd.
So yeah, straight up lies and ignorance what these people are spouting, and they should be ashamed of themselves for their conduct.