After thinking about it for a minute, I don't have an opinion on your comment either way, but I think it's a good question to ask.

This control is more concise than two controls, which is nice especially on mobile. But, we could easily hide the button with animation, and replace it with a classic loader. Can we say anything about which approach is better, without empirical data?

I excitedly up-voted the post after clicking a few buttons, and I think the author is a creative and clever person either way!

Once the user clicks "Submit" you have to disable the button to avoid double submission, so might as well replace with a progress indicator.