Not to dilute the achievement, but looking on the F3 Cup website…
“The pair will run to F3 Cup rules on tyres, ride height and fuel, although they will compete in an invitation class as the championship caters for cars built from 1980-2007.”
That last bit suggests they are running newer cars than the rest of the field.
If that’s right, it says more about Kevin Korjus (only making 3rd) than it does De Costa!
Can’t wait to see De Costa in F1.