I may be corrected but I have heard kammatthana means 'object of work'.
Kamma here means 'work' such as in the word 'kammaniyo', which means 'ready for work' or 'active'.
Buddha said one quality of samadhi is it is active or pliant. It is sensitive and flexible for the work of introspection or vipassana.
So evaṃ samāhite citte parisuddhe pariyodāte anaṅgaṇe vigatūpakkilese mudubhūte kammaniye ṭhite āneñjappatte āsavānaṃ khayañāṇāya cittaṃ abhininnāmesiṃ.
When the mind was thus concentrated, purified, bright, unblemished, rid of defilement, pliant, malleable, steady & attained to imperturbability, I directed it to the knowledge of the ending of the mental fermentations.