Instead, seeing the word if he's cast the clause in a form that he believes accords with a grammatical rule.

I realised my mam had taught me something more valuable than any grammatical rule.

Does this change the grammatical rule about case of pronouns?

It's a case of hypercorrection, in which the writer has mechanically applied a purported grammatical rule where it doesn't apply.

When some grammatical rule became changed or disused, some verbs kept to the old pattern.