Project-based billing solves this neatly. It also ensures alignment of incentives (ie, working faster / more efficiently yields higher effective rate, vs working faster -> get paid less), and is helpful in emphasizing value.

For time and materials projects, avoid hourly if possible; that leads to micromanagement and commodification of your skills. I recommend billing by sprints or by the week. Try to limit granularity to a day rate (w/ minimum increments of 1/2 day).

It's better from the client POV too. As a client, I have work that must be done, I want to know how much it will cost, I don't care whether it takes you 3 hours or 10 hours, and I especially don't want to hear "whoops... I miscalculated, I will need more time, gimme more money".

So many consulting companies would bill those hours anyway

Bill by FTE day/week/month instead. Much better for your nerves and easier to invoice.

What's the difference? Billing for any of these seem equally dishonest.

What is the argument that goofing on HN is more justifiable to a client than walking and talking?

I believe this is billed as "Executive Creative Time" in modern nomenclature.

Charge for the project. "You logo will cost $5,000"

Hourly billing is for chumps.