What I can't tell from that story is, have they been working for the Phillies for free, essentially, from 1984 to 2018, having been paid a lump sum for creating the character, or was that work paid separately? Not that it probably matters to the fundamental question.
“For more than 40 years, we have worked closely with the Phillies, making all the Phanatic costumes, providing artwork and ideas until June of 2018.
I feel like this could backfire on the Phillies, their intent seems to be to establish leverage by saying, "instead of paying you to use your character we're going to change the character enough so that it's not yours," thereby forcing them into a settlement. But the message it actually sends is, "we concede the character is yours so you should probably hold out for more money or a trial."