I have to disagree with you.
It does not make any sense when the same writer would write in one sentence: She sat in the chair, and on another sentence: She sat on the sofa.
At a library I found a book with instructions for those who wants to write a book. The first thing it says: Do not write: She sat IN the chair. To sit in the chair, someone has to brake the upholstery or wood. I also found in the dictionary: Sitting on the chair.
On one thing you will agree with me: Not much grammar is taught in American schools. Therefore, so much confusions and not much knowledge.
I have no issue with saying 'she sat in a chair'. For me, it depicts a compfy chair with high sides and back. Sitting 'on a chair' depicts a chair with a flatter profile, like a sofa possibly.
It is said that those learning a foreign language will struggle to understand the second meaning of the language, and I think that this is a classic example of that. 'In' and 'on' a field are also both feasible with slightly different connotations. It's a tricky subject, and I would guard against pronouncing different versions as errors. Judging by its first piece of advice, I think I would throw that book out if I were you.