From the book:

In one respect this is a return to the status of the consulship in the pre-Sullan period, when major wars were fought by consuls, not pro-consuls with long-term commands.

Sulla was a dictator who effectively took over civil and military rule of Rome, suspending the Republic, and causing consternation amongst the Senators, patricians and other oligarchic members that he wouldn't relinquish his dictatorial powers. Sulla did. But political control had been recast and now came with the sword and dictatorship.

This is the problem with a non-written constitution it leaks constantly as new constitutional precedents and conventions are established with each successive emergency. It ends up being hacks upon patches which inevitably get used for other than what they were originally intended.