"The present moment is where stewardship is testable."

This sentence means:

We cannot directly observe how people governed a territory 2,000 years ago.

But

we can evaluate

how today's government governs today.

For example,

today we can examine

• public services,
• courts,
• elections,
• protection of rights,
• infrastructure,
• security.

Those are observable.

That is why he says

stewardship is testable.

He is arguing that current governance can be evaluated directly, whereas ancient governance often cannot.

Notice,

this is his political principle.

It is not a rule of historical method.