I have made note in your question as to the actual location of the quote. It is crucial to analyzing the quote to understand where in the play Lear makes this observation about himself. Is he reflecting after long struggling and learning of lessons about life, or is he reacting in a self-pitying, momentary reaction to an immediate event?
The context of the quote is the middle of the storm on the heath in Act III as Lear is condemning those who have committed crimes (his daughters) and proclaiming that they be brought to justice. His condemnation and finger pointing here is in direct contrast to later (IV, vi) with Gloucester, when he is much more contemplative about his situation, observing that:
See how yond justice rails upon yond simple thief? Hark, in thine ear. Change places, and handy-dandy, which is the justice and which is the thief?
But here in Act III, he is still vengeful and desires justice/punishment for the crimes he believes have been committed against him. Later, in Acts IV and V, he sees more clearly how interchangeable the idea of "guilt" is and also understands he himself to be a "foolish, fond old man."
So, it seems to me that the question of sinned and sinning is merely in the eye of the beholder, and, for Lear, it depends on the perspective he has (or has not) about the common human nature we all share, including being sinners.
This is a wonderful question.
In his own mind, Lear is definitely more sinned against since he was betrayed by his own daughters.
He does not see his own "sins". Probably his greatest sin is his treatment/rejection of Cordelia who truly loves him but he is blinded to it by his own ego.
It is only when he is thrust out into the storm and suffers along with his companions that he begins to understand humanity. As a king for most of his life, he could have sinned but because he was a king, it didn't seem to count since kings were supposedly anointed by god. For the first time in his life, Lear is a victim of a power greater than himself. And for the first time in his life, Lear is afraid.
Only god....or Shakespeare can really answer this question.