Compete sanctification. That we always walk in faith, therefore always do God's will, that's what I mean.
Thanks for clarifying what you mean by the question, or "sinless perfection." Working from your definition. I can agree that one is able through the grace of God, not to
"willfully transgress a known law of God." In this, it does not make one perfect in knowledge, nor does it make perfection of judgement, for one could innocently "borrow" a pen, get home and not know that they actually stole that pen, or even remember who and when they stole that pen. There is sin as a act or intent (knowledge) and a sin of ignorance. In Scripture, God does not demand that we are to be equal to Him in perfect knowledge, and performance. That would be absurd; it would make a definition of sin to be utterly unavoidable, even by breathing imperfectly. Scripture says that we can avoid sin, so if we cast the net so broad, many passages become self-contradictory or impossible commands. If we have a good Biblical working definition of sin, it will not contradict many of the 41 instances if "sin" as a verb in the New Testament.
I do not know the person or the context in which the person you spoke of as claiming to be
"sinless." Possibly, he was not working within the definitions of "absolute perfection" as if he were God himself, but knows that he has not
"willfully violated a know law of God." Now, I would never use the term "sinless" to describe that state, but to use a Biblical term of "does not sin." There is a difference in Scripture between not sinning (properly defined) and absolute perfection. To be clear, one can have a perfect motive, and perfect obedience to what they know is right and wrong, and still have error in judgement. You speak of "progression," which is different from "maturity." Purity and maturity are different. One can be limited in knowledge, yet be pure. Maturity takes time and progresses. Just because God purifies and sanctifies and individual, they can still be completely obedient to God as far as they know, yet God may convict them of that sin at a later date. They were not "sinning" in the realm of knowledge and rebellious action, yet they may have been violating something that God said was sin, but they did not know it. The issue is knowledge and motive, yet I would never say that those so-called sin, or violations did not need the ever-present Atonement or Jesus Christ to keep them reconciled to God. No one, in and of themselves, can purify their own hearts apart from the Holy Spirit working that grace within them.
I knew a man that smoked, even after becoming a Christian. Other believers heaped judgement and condemnation upon him, and yet he felt no conviction that it was wrong. A couple of years later as he prayed, God struck him with the conviction that it was "sin" for him. He never smoked another cigarette. Was he "willfully violating a know law of God" before that? No! Would he have sinned if he refused to obey God's clear conviction for him? Yes!
I cannot speak for others on the issue, but in established doctrinal history, this is what most people mean by "not sinning," sanctification, or Christian Perfection. As for "sinless perfection" and what people seem to mean by that, it does not resemble anything that I know of as a real doctrine that anybody has ever taught. That is why I consider it a strawman argument.
I have never seen or met a person that thought that they were God, perfect as God, or never sinned in the sense of absolute Divine Perfection. Ironically. however, I do see those that claim that they
do not sin in spirit, and that sin is only in the flesh, when applied to Christian doctrine, is nothing more that sugar-coated fiction from the heresy of Gnosticism.