G. White wrote:
"This example is intended to be illustrative rather than exact."

Nobody contradicts that maximum power from an amplifier requires an
impedance match, so far.

Nobody contradicts that efficiency in an ideal Class A amplifier can
never exceed 50%, so far.

Some texts show waveform distortion typical of a single-ended Class A
amplifier and note that "maximum undistorted power" requires a load
impedance 2 to 3 times that of the amplifying device.

G. White gives a peak to peak voltage of 20 and a power out of 5 watts.
RMS volts are 7.07 in this case. The load then is the square of the
volts divided by the power. This is 50/5 or 10 ohms for the load. If
this matches his amplifier, 5 watts is the maximum output.

G. White said that he did not consider the output-Z of the amplifier in
its loading. Ignoring an amplifier`s impedance does not make it go away.
Ignoring an amplifier`s impedance does not revoke the maximum power
transfer theorem, either.

Best regards, Richard Harrison, KB5WZI