You don't need a test for every function. You probably want every function call covered by a test, though, otherwise you have untested code.

The exact granularity is a debate that has gone on for a long time. Nowadays, people seem to prefer larger tests that cover more code in one go so as to avoid lots of mocking / stubbing. Super granular tests tend to be reserved for libraries with no internal state.

While what you say could be argued, this is both an insufficient argument against, and irrelevant to, the post you’re commenting on.