This is incorrect. iTTL relies on TTL metering only, the flashes don't
meter themselves in this mode. So his softbox setup would be safe.

Here's a bit I stole from a website, explaining the process:

"Nikon has long been known for their 3D matrix metered fill-flash
capability, and the exceptional ease it brings to fill-flash
exposures. With iTTL (the "i" is for "intelligent" TTL), they're
further extending the capabilities of their flash technology. For the
first time, iTTL makes use of the 1005-element RGB sensor used for the
main exposure system, the accuracy of the metering pre-flash has been
improved, and wireless capability has been dramatically expanded, with
the implementation of true multi-unit, TTL wireless flash
autoexposure. To me, that's the most amazing part of the new flash
technology: Not just that it provides automatic flash operation
without wires, but that it offers true Through The Lens (TTL) metering
for flash exposures, even with multiple remote flash units: With an
SB-800 speedlight as a master controller, up to three separate groups
of SB-600 and SB-800 strobes can be controlled independently, each
group consisting of an unlimited number of units. The D70 itself can
also serve as a master controller, although it can only control one
group at a time, and its internal strobe can't contribute to the
exposure when it's acting as a controller. (Note though, that if you
mount an SB-800 on the D70's hot shoe, the SB-800 can both act as a
master controller, as well as contribute to the exposure itself.)

When I first heard about it, the whole wireless TTL system sounded
like just so much magic, but it's actually pretty straightforward once
you know what it's doing. (Straightforward, but no doubt requiring a
lot of clever engineering.) The key to it all is strobe circuitry that
can turn on and off very quickly, and fire multiple precisely timed
bursts in a very short period of time. The iTTL system uses this
capability in two ways. First, it uses rapid series of very brief
pulses of the strobes to let the Master Controller "talk" with the
various groups of remote units. The Master can command groups of
remotes to fire either very brief pulses for metering, or more
powerful flashes for the exposure itself. It does this by sending
treating the flash head as a digital data channel, encoding commands
about the type and intensity of pulse to fire in the form of rapid
bursts of light.

The second way that the fast-pulse capability facilitates iTTL is by
making it possible to determine exposure levels from multiple flash
groups very quickly. (The speed is important, as you don't want the
flash exposure determination to introduce an unacceptable shutter lag
when working with multiple groups of speedlights.)

The way the iTTL system works is that the camera tells the Master
controller to individually command each group of remote flashes to
fire a metering pulse. Using its internal TTL sensors, the camera
measures the amount of light coming from each strobe group, and
integrates the light readings from all of the strobes with the ambient
light coming through the lens. Via the Master controller, it then
tells each strobe group how much light to emit for the exposure
itself, and triggers them to fire when the shutter is opened.

If this sounds like a lot that has to go on before the shutter opens,
that's because it is. It all happens very quickly though, without
introducing an appreciable delay in the shutter release. (If you have
several groups of strobes involved in a single exposure, and have
quick enough visual reflexes, you can actually see a very brief period
of flickering strobe flashes before the main exposure itself.)

The results are really pretty amazing. You can more or less scatter
strobe units around the set any which way you want, and the iTTL
system will deliver not only an accurate default exposure, but perfect
control over the light being delivered by each group."

Source:
http://www.imaging-resource.com/PRODS/D70/D70P.HTM

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Owamanga!
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