Angel,
you are forgetting that this is a push-pull amplifier.
If the MOSFETs were perfect (zero saturation voltage), then the
available voltage swing would be ±13.8V for each, which equals 9.76V
RMS. Between the two drains that's twice as much, so 19.5V RMS. This
voltage applied to 50 ohm gives 7.6 watts.
Real MOSFETs do not swing fully to ground, but an IRF510 on 80 meters,
at such low power and current, gets pretty close. So the maximum
unsaturated output might be 7W. The author's spec of 5-6W seems
conservative compared to that.
In saturation that amplifier should produce significantly more power,
but of course it's no longer linear when driven that hard.
Manfred
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