Absolutely idiotic. You misunderstand a lot of simple principles. You push into the ground with a punch. Itís Newtonís action-reaction that generates the force, exactly the same as a sprinter. Yes you stay on the ground but itís the same manner of generating force.
Your inside vs outside muscles thing doesnít make any sense. I assume when youíre referring to Ďinside musclesí that stabilise youíre talking about the multifidus and transverse abdominis? Yes stabilisation aides in transferring power but itís the large muscles (or outside muscles that you strangely like to call them) that are responsible for pure force, for global strength.
The core (which Iím not sure you really understand) is strengthened terrifically by heavy squats. In fact every heavy weight exercise requires maximal activation of stabilising muscles (Inside muscles), so a heavy squat is going to be far more beneficial for your core than some pointless sit-ups.
Technique is the number one thing for punching power but if each of the links of the kinetic chain are stronger then force generated is increased. Just because an exercise isnít completely sports specific doesnít mean that it doesnít have benefits that are transferrable.
Heavy weights increase neural activation and decrease inhibition from afferent receptors, allowing more force to be applied.
And the number one benefit to weight training is the effect it has on the tendons, tendons are responsible for power. Heavy weight training increases the stiffness of the tendons. A stiffer tendon is harder to stretch but recoils at a higher speed. You canít move without tendons, you need high muscle strength to stretch the tendons. A stiffer/thicker tendon produces far more power than a compliant (loose, thin) tendon. Go pick up a thick rubber band and a thin rubber band and stretch them. Thatís essentially what your tendons are.
Itís unbelievable the ignorance that gets passed around as expert opinion on the internet.