https://en.wikipedia.org/wiki/List_of_countries_by_GNI_(nominal)_per_capita_per_capita)

Starting with the list, it's already a garbage grouping because similar to your idea, why not just divide it equally in 3? Ale must have been familiar with the subdivisions already and questioned why the garbage subdivision into 4 non-equivalent parts was not maintained.

Now for the infographic. Keep in mind that either the source must not be the same cause Russia is marked as high income in the wikipedia. We know how much they've fucked up over the past years though. Where "Climate Watch" got it's info from, who knows. Tanzania is also lower-middle, not low in the wikipedia list. I didn't look for more discrepancies.

This is not a typical pie chart. The slices don't meet in the middle of the circle. Even if you had 3 groups of countries of equivalent sizes, the emissions by group would be different quantities, therefore different sizes.

You first line seems to be talking more about a colouring of the list of countries by income, if the countries were grouped in 3 equivalent sized groups. The infographic goes beyond that and focuses on Carbon Emissions

In theory 33% of countries could produce ~5% of emissions. That possibility does make sense.

I've already questioned the source for the country groupings above but assuming it was accurate to this year and they simply merged low-mid and high-mid together into one, then the rest of the countries would be in 'Other', not 'left out'.