What was changed into an immortal object?
As you say yourself:


  Was this an error, in that the message was supposed to refer to this boss removing his own immortality? If not, what exactly did this message signify?


While googling, I came across some posts on a forum which seems to say that it is a wrongly translated message and that it should say something like: "immortality status changed".
I'm not sure if it's accurate but it seems to tell the exact same thing.