How hard would it be for DO to change the sign up button to 'Sign up/Login'? Seems like an utterly trivial change to make for something that's completely possible, but totally hidden, while the 'visible' solution is extremely aggravating.
That's a textbook example of poor UI.
I've seen a lot of rhetoric, even on HN, that frontend/webdev is "not that hard", that frontend/webdev engineers are doing busy work to justify their jobs, etc. And then people with this mentality go to their jobs, they subconsciously or overtly promote these views in their workplaces, people who are good at this stuff try to avoid toxic workplaces that undervalue them rhetorically or with actual compensation, and then you wind up with "easily" fixable things like this and the toxic people blame the frontend engineers who are willing to stay in environments like this.