Did you board late or were you boarding with SkyPriority? If the latter, then I say the poacher had malicious intent, you don't get to ST+/GM+ without knowing your seat assignment.
As for the process, you do raise an interesting question. I'm one to always want the seat on my boarding pass, especially if it's a C/C+ difference. But iif my seat had now been occupied then the cleaning that Delta performed would no longer be 'fresh'. On the flip side, the passenger in the window of my row may have touched the arm rests, passengers walking by may have touched the seat back for support, etc.
If I had to fly, then now that you ask it I'd probably bring some clorox wipes in a Ziploc for a quick wipe down even if nobody had poached my seat.