You make a good point, but to be clear it’s only the second part of your point that is flawed. Finding a biological cell on Mars would most certainly be definitive proof that life existed there. I am pretty sure that is extremely unlikely to occur. Evidence for life in these samples more likely would consist of finding biochemicals such as amino acids, nucleus acids and the like rather than intact cells. Currentl conditions on Mars are pretty inhospitable for life.
Current conditions on Mars are inhospitable to amino acids. Why did scientists choose “amino acids” as the precursor for life? Because it was the only reasonable jumping off point they could find outside of a laboratory. Abiogenesis theory is classical backwards reasoning. Life HAD to start somehow, so let’s pretend about mud and lightning and invent word salads like primordial soup.