The problem is the term 'pantser'. It clearly means different things to different people. It does seem to be equally applied to two different approaches - one based purely on looking back, and one based on writing without adhering to a series of predetermined events. Here's where people seem to get argumentative.
In my case, I start writing with very little idea of the details of what's ahead, and I'm open to working with whatever presents itself. For example, when I started I had no idea that my MC would have a fight with his wife about infertility, but - as I was working on the previous scene, it occurred to me that that would be a very useful way to increase the tension and complicate things for my MC. Now, I got the idea that in this fight, she might call him by his full title. It would give that line a lot more meaning and impact if the reader knew that it was exactly his full title, and to make that happen I'd have to go back an insert the title into the text. But I still think that I am using a more spontaneous form of story composition than making up my story by determining the events of the story before I begin the writing of the text.
But since 'pantser' includes both those who only look back to write the story, and those who eschew determining the events before, and separate from, writing the text, it really is meaningless to say that one is a real pantser or not. Myself, I really like the term 'lookback' for the approach of considering only what has happened before to determine what happens now. (If you were publishing serially you'd of necessity be limited to that.) You can definitely say that not all pantsers are lookback writers.