The Nikon metering system is balanced fill flash.

Balanced flash means the flash level is adjusted to lower level to not overexpose the area also illuminated by the ambient metering.

I don't have a Z7, I just looked in the manual (PDF version is text searchable). I do have other Nikons.

Z7 manual page 351. Last sentence: "Standard iTTL fill flash is activated automatically when in Spot Metering is selected." This same is true of the Nikon DSLR models too.

The SB-600, SB-800, and SB900/910 flash units have a switch to select balanced nor not balanced flash, but 3 of the 4 are discontinued now. The cameras have no way to select non-balanced other than to specify Spot Metering. Seems poor to me too. The camera needs that switch or menu.

That has more meaning. The Flash is NOT spot metering, period, never ever. The flash metering system has its own larger central spot it always uses. But the ambient is spot metered, if selected. And fill flash metering is not compatible with spot metering, different concepts, so the flash still does flash metering in that mode. Says there, this Spot mode means the background illumination is not taken into account (by the flash metering). Meaning, not balanced flash.

Indoors, where we use flash, the ambient is low and will be underexposed, so it really does not matter what it meters, or if Spot or not (the camera settings are set for the ambient metering, but then in dim light that metered slow, the Minimum Shutter Speed With Flash increases to a faster minimum, underexposing ambient... E2 menu, page 264). Meaning, with flash detected present (communication), all indoor flash pictures will likely be 1/60 second shutter (or faster in brighter light), regardless if dim ambient actually metered slower. Slow Sync mode will use the actual metered ambient shutter speed, or possibly E2 menu can allow it. But we are using flash instead.

So Spot metering will normally have no effect on ambient indoors with flash, which is normally where you want non-balanced flash anyway. It is just a very awkward way to get it there.

But spot metering will have very great effect outdoors in sunlight, so you absolutely need to understand spot metering there (it does NOT mean the spot is correct exposed, it means the spot will come out middle gray, whether it should or not). And really need to remember Spot might be on when going out the door.