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Oxford Dictionary 5 (https://www.lexico.com/en/definition/just) gives this definition about "Just"
Expressing agreement.
and gives this example
A: “Simon really messed things up.” B: “Didn't he just?”
Does the guy B mean Simon did worse than messed things up?
No, Guy B means "You're right, he did."
The question is entirely rhetorical--a way of expressing emphatic agreement. These are pretty much the same, and the "just" primarily serves as an intensifier of agreement.
Person A: The movie was terrible. Person B: Wasn't it? I hated the plot and the characters.
Person A: The movie was terrible. Person B: Wasn't it just? I hated the plot and the characters.
This usage of just is somewhat related to the meaning of just as "exactly" or as in "just so."
You could interpret the question as "Didn't he [mess up exactly as you say]?"
