I wonder if there's a "control group": a tiny but identical container that goes through all the same steps. Nothing can ever be proof that it wasn't contaminated or a seal was broken on the real but not the control container, like, nothing will convince an Internet conspiracy theorist, but a control vessel and a handful such missions ought to prove beyond reasonable doubt that it wasn't contaminated from earth. I'm personally already willing to trust that they thought of (and controlled for) these things and more, since it's their job and we're just thinking out loud here.