I assume it's my colorblindness, but on mobile when I move that slider back and forth I just see blue outlines appear around the countries with no other distinguishable changes.
I am not color blind AFAIK, and I had to move the slider back and forth many times to try to understand what they are trying to show (even though I kind of knew what to expect).
Not the best visualization I should say.
The colors could have been better used to highlight the difference IMO.
The colors and the sliders are both a bit extraneous to the point of the visualization, they're supposed to help but they kind of obscure the point.
For the northern landmasses, Greenland for example, the smaller outline is the equal-earth-projection, and the larger outline is the conventional Mercator projection. That's all there is to it.
I just viewed it on desktop, and here I can see the inner and outer outlines. On my phone I studied the diagram closely, but can't see the inner map outlines. Perhaps just a color profile thing on my phone.
Not the best visualization I should say.
The colors could have been better used to highlight the difference IMO.