It is how it is described -- but your instinct of a problem is right. f = ma ; a = dv/dt ; v = dx/dt
Without a change of position of some mass in a period of time there is no force. No physical movement means no acceleration means no force.
But I know what YOU mean -- the person moves in the zone of stability defined without changing the base of support. In many earlier discussions "not physically moving" was obstinately undefined by some who insisted on using that description. The nature of "base of support" should be also specified in a given case to remove that source of needless ambiguity (which you do in general terms).
Just to be clear, the infamous "Teacher Test" was not a completely 'no movement' test. What I asked was that someone claiming to be a teacher (hence "teacher test") of "internal martial arts" put his hand on my and hit me as hard as he could *without pulling back his hand or his shoulder*. Of course someone who uses his dantien/hara for power instead of his shoulder, etc., as a bona fide teacher should, would have no real problem with this simple demonstration.
Ultimately, at higher levels, the ability to generate a lot of force with only a small motion is conforming to the old sayings about "motion approaches stillness", and so on. Although the equation of F=ma is always valid, I'd suggest that the variations of that equation which have to do with Impulse and momentum are worth thinking about, also.