If it had enforceability, they would explicitly cite laws to that effect in the subpoena itself. If the subpoena does not have that, it's almost assuredly unenforceable.

Subpoenas, generally speaking, do not require a signature of a judge and aren't subject to the same scrutiny as a warrant.

Subpoenas are issued against third parties (in this case Google) to compel the release of information. Google can fight the subpoena in court, but they generally don't.

The enforceability only comes if Google decided not to comply, at which point they could offer to sell the information to the government or a judge ruled that Google must comply.

Again, most companies comply because they often lack the incentive to not comply.