You are making an assumption that isn't valid.
Some seats in the exit row have less leg room because of equipment stored under there and some seats in the bulkhead and exit row have other restricted space. Seat 15A has a short seat and arm rest - maybe to make room for people exiting the plane in an emergency.It definitely would not be as comfortable as the exit row.
No question in my mind - Coulter was targeted. I'd bet good money the people who got the seat she paid for were non-rev family members of some flight attendant.
People who fly a lot always check the seating maps and the internet forums to choose their seats. 15A is not a desirable seat.
I might have.
I am not a cheerleader for Delta.
I choose to not fly with them based on my past experiences with them.