You're overfocused on the "technology" and underfocused on the base rate of the cancers you're concerned about. Just do the math. What you need to know is "just how accurate would this test need to be in order for most of the positives it generates to be true positives". The numbers will be surprising.

Both the technology and the base rate DO matter.

Say that you are hunting the elusive snipe, one bird in a million. With standard techniques, you will have a lot of false positives.

But if you learn that the elusive snipe gives off a weird radio signal that other birds don't, your hunt will be a lot shorter.

Same with relatively rare cancers. If you can detect some very specific molecule or structure, your test will be quite reliable anyway. That is why I don't get your use of "probably". Unless you are really familiar with the underlying biochemistry, the probabilities cannot be guessed.

There is absolutely no reason why tests for rare diseases should have high false positive rates. In many other diseases, they don't. For example (although the underlying technology differs), Down syndrome is rare, but its detection barely has any false positives. You can test the entire pregnant population for Downs reliably, and many countries already do that.

I'm not saying that you couldn't develop a super-reliable test for a rare cancer! The point isn't that these cancers are impossible to screen for. The point is that the numbers given in this article do not suggest that such a test has been developed in this case.

It surely does not make sense to screen the entire population for pancreatic cancer, but we already do have a screening for pancreatic cancer in Czechia for "at risk population".

https://www.cgs-cls.cz/screening/program-vyhledavani-rakovin...

For such programs, a blood test would be a huge boon and they could even expand the coverage a bit.