They aren’t really talking about the copyright in this context. Also artists can usually still claim the work they did with a studio as their own and show it to others at their discretion, they in fact usually put their work in their portfolio to show to potential future employers, or other animators, or even just an online following. Spencer Wan for example likes to post the line tests/animation tests/keyframes/storyboards/etc for some of his animation sequences on twitter, and has done so for Castlevania, The Owl House, Spider-verse, etc. Animators just can’t sell the same work for profit or provide the completed project in its entirety, at least not without permission. There may be restrictions on how early they can show their work publicly depending on the contract, but that’s usually to prevent major spoilers or leaks, and varies between studios/projects. Also, they still get credited for their work in the project they worked on in the credits, so it’s not like they would be completely divorced from it even if they weren’t allowed to share it separately. It’s still classified as “their” work.