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:
select CASE WHEN CAST('{"a":null}' AS JSONB)->>'a' IS NULL THEN 'yes' ELSE 'no' END;
Oh, an CAST() may look like a function call, but it is also a syntactic element. I.e. there is not a function called "CAST".
Well, I messed that up a bit. CAST is not a "syntactic element" as I was thinking. But it's not really a function call either but is closer to a function call than an operator in nature (I my ignorant opinion) . I was thinking it was a "compile time" operation, but it, like a function call, is a run-time operation. I think that using the CASE() makes it plainer that it is _not_ an operation like ->> or - and makes the precedence issue plainer.
--
Schrodinger's backup: The condition of any backup is unknown until a restore is attempted.
Yoda of Borg, we are. Futile, resistance is, yes. Assimilated, you will be.
