I believe their logic is such (I’m not involved with the study, but have a background in medicine and research):
Elevated PSA (a blood test) signals prostate cancers.
PSA tests are relatively routine bloodwork with an assumption of uniform coverage across all patients, trans or not.
PSA tests are presumed to uncover early cancer presentation.
Therefore, if we’re only seeing advanced cancer presentation in trans women, the PSA test is a poor screening device for early prostate cancer in that population.
Point 2 is a big assumption; I am ignorant if that would be a confounding variable in real life, or if that’s even been studied.
PSA == Prostate Specific Antigen, referring to a blood test used to screen for prostate cancer.
I presume the rest of the words are easily understandable or able to be looked up.