No, you're not missing anything - that function is not present because it was forgotten.
IMO browsing by 'All' (and then blocking communities) is a crutch for the poor community discoverability on Lemmy. If finding communities was easier then you'd subscribe to the ones you want and then you wouldn't need 'All'. I have tried make finding and subscribing easier by using Topics and including topic subscription in the onboarding process but I admit it's not a complete solution.
It would not be difficult to add a community block function. I've just added an issue for it https://codeberg.org/rimu/pyfedi/issues/283