Validity of Cuts Without Going Forwards or Backwards
Is a strike valid if I cut without stepping forward? Example, an opponent steps forward and attacks my men, but missed. Then I hit his men but did not step forward. Is it valid?
This is very hard to answer because a valid strike (yuko-datotsu) is not only achieved by hitting a target.
All Japan Kendo Federation defines yuko datotsu as the accurate striking or thrusting made to targets (datotsu-bu) with the monouchi of the shinai with a sufficient kiai, the right posture, and zanshin.
It does not say about stepping forwards or going backwards. So the strike you described above can be a valid strike. As a matter of fact, I have seen such strikes became valid.
The strike you described must be as strong as the ones with fumikomi. You have to have a good tenouchi to do so.
Long strong short. It is possible but it is hard to be valid.