。
As if that were proof of correct English.
Literal
Exactly-as-if that [subject-が] correct English [possessive-の] evidence [as if-かのように].
まるで…かのように 'exactly as if' — a simile pattern expressing something counterfactual. 証左 'evidence, proof' is a formal/literary word (more common in writing than 証拠). The fragment feels like the trailing end of a longer argument — a sarcastic dismissal.