I believe the reddit API might not allow full discovery of comment history. At least my experience with deletion tools was that once I had the data export to check I found only a small portion of the posts from by 12 years of history were deleted despite the reddit UI and deletion tools not showing any comments remaining.
I had to use a tool to go through the GDPR export to find all the posts and the tool has had problems due to some subs being private due to protests. I suspect a lot of people who thought they deleted their entire comment history may not have done so.