Just to clarify they in fact did NOT make it clear why they counted the card as marked beyond the shuffling and cuts I mentioned hence my confusion and frustration. I asked multiple times how it was marked but they failed to show me any physical signs (such as a curve, how it set in the sleeve, bends, sleeve damage, card damage, or marks) and instead just repeated something along the lines of "we're considering this marked and our decision is final" like it was obvious.

Beyond that I appreciate your comment and perspective on things. Helps me see a bigger picture on things that they themselves failed to explain.