It's been a long time since I saw Blackfish but I don't recall the focus of the film being "SeaWorld should release all its captive orcas into the ocean."

You're absolutely right. That user's so-called "well thought out comment" is not only arguing against implications and/or claims that Blackfish never makes, but the documentary actually does cover various reasons why orcas in captivity wouldn't be successful if reintroduced into the wild. (E.g., at one point they even spend like ~15 mins talking about pod language, and why orcas can't form proper social bonds with each other in captivity or if released back into the wild.)

The only theme in Blackfish dealing with the notion of orcas in the wild is the numerous points they make when comparing wild orca behavior and physical attributes with those in captivity — such as the dorsal fins and teeth. IIRC, there's never any call for SeaWorld to release all their orcas into the ocean, rather just putting a deadline on their orca exhibits by not acquiring anymore of them.