This is so common nowadays that for many sites I have the direct login page bookmarked. It indirectly implies that once you sign-up, the company stops caring about you. At-least for me.
I was able to directly link this page however, which I bookmarked:
I don’t understand the problem, if people want to try or sign-up for your service they’ll locate the signup button. That’s a one time problem. A hidden login button just annoys existing customers.
My feeling is that this is down to testing without privacy in mind. Your site might be fine, but others aren’t so a minority of users will clear cookies at the end of each browser session. That’s not a senario most will test for or experience.
It seems rather straightforward to me, from their example, to de-emphasize the "Sign Up" button and prioritize the "Sign In" button for someone who already has an account.
1. Less likely to abandon your service than prospective users
2. by far less likely to even see your landing page, since they’re usually already logged in.
In this specific context, your existing users are the less important ones.
It's otherwise very hard to convey that you care about existing customers so this seems like a no-brainer.
Perhaps these are similar symptoms.
precisely