Looks correct to me. As I understand it the ::jsonb is NOT an operator! It is a syntactic construct for a CAST(). An equivalent which might make more sense is:
My issue is nothing to do with the ::jsonb cast, it's the precedence of the ->> operator.
As far as I can see the ->> operator has predence below '- +' but above 'IS', but there's no entry for it in that place in the precedence table.