You didn't misunderstand. Signatera is definitely intended to pick up a recurrence before a CT scan or symptoms. And it definitely does in some cases. (It can also be used for other purposes, like tracking whether a treatment is working). But it is still an open question what fraction of recurrences can be detected early this way, particularly for cancer types like uterine clear cell carcinoma that have not been studied with this test.

If you read the company's website, they make it sound great, but my feeling is that they're over-selling the test's ability to detect really low levels of cancer. You might want to read other people's opinions about it by searching "Signatera" on Mayo Connect. In particular, there's a post and a number of comments about it on the pancreatic cancer board.