My question is why 2Zo rather than just Zo. Is it because only half of the
power
in the field can be extracted and delivered to a load?

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Yes. With the current induced in the receiving antenna it doubles as a
radiator and re-radiates half the power back into the field. But only when
the receiver and antenna are Z-matched.

If not Z-matched then more than half of the energy is re-radiated and less
than half of it is accepted by the receiver.

It's a simple case of a conjugate match between a generator and a load which
applies whether the antenna is resonant or not.

In series with the antenna's radiation resistance there's a small conductor
loss resistance and a corresponding small loss in power efficiency. The
radiation resistance changes when in an off-resonance condition and so does
efficiency.

But apart from the change in efficiency, provided a conjugate match is
maintained, the 50 percent receive/re-transmit condition always holds and
even when well away from resonance a receiving antenna can have high
efficiency.

It is required only that radiation resistance be considerably greater than
conductor loss resistance. Which is rather obvious whichever way one looks
at it.
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Reg.