There are still testing backlogs for COVID, it can take multiple days to get results (assuming the doctors decide to run the test). Link #2 also says: It’s hard to know for sure [if clotting is a major factor], because the clotting problem is apt to go undetected.

Since most people think of COVID as a respiratory disease and the clotting symptoms aren't as well publicized, doctors may not always call for the test when respiratory symptoms aren't present.