In most cases, the subjunctive form of a verb is usually the third-person form of the verb with the ‑s dropped, but the verb to be is a special case. The subjunctive is used after certain expressions that contain an order or a request, a hypothetical, or a wish.
The phrase "a hypothetical" being used as a noun phrase there.
According to Cambridge Dictionary, "hypothetical" is an adjective and the noun form of it is "hypothesis".
Is that a typo or some kind of grammar rule?