Why weren't all surviving Viltrumites DNA tested?

Anissa says "You found the heir of Argall?" And the second Thragg confirms he did all the viltrumites attack him.

So did they all know to be looking for Argall's heir?

They had less than 50 viltrumites and didn't all get tested for Argall's DNA? I don't get it

Could Thragg have been testing viltrumites privately and killing anyone that had Argall's DNA?

Did they all assume they weren't related to Argall?

If they test the remaining viltrumites and there are no matches then they can move on and make whoever they want Emperor. Or they can find a bunch of heirs and they can fight it out. I don't know. Thoughts?