That's right, there's very little crossover between when the bulk of Sounder passengers would arrive at the station and when Amtrak trains call there. The only Amtrak train to stop at Tukwila during morning rush hour is 501 southbound to PDX, which I suspect draws few passengers at Tukwila on any given day.
The only problem with this enforcement strategy is that it's very personnel-intensive. This isn't a case of sending someone around once a day to check slips on a dashboard. You have to essentially catch someone in the act. Not sure that's sustainable in the long run, but I suppose if you used this enforcement once a week or so, it would send a message to the regulars there. [Of course, the message might be: Feel free to park in the Amtrak spots if you don't see any parking enforcement officers]