I agree with the first part of what you said.

(Note: I am an amateur Nutritional Geneticist)

They said:

"The results demonstrate that carriers of the genetic variation have what is known as sucrase-isomaltase deficiency, meaning that they have a peculiar way of metabolizing sugar in the intestine. Simply put, they do not absorb ordinary sugar in the bloodstream the way people without the genetic variation do. Instead, sugar heads directly into their intestine."

Ha! What? SI, the gene that they studied, ONLY exists in the intestine. The inability to break down sucrose leads to microbe imbalance that concerts the sucrose to acetate. They are using the word "sugar" to cover everything from sucrose to any other oligosaccaride. They just do not understand the science enough. If you read the study it is clear.

And in NO way doe sthis mean that eating sugar is healthy for them as the title implies!!!! Look at what happens when you have this deficiency!

https://medlineplus.gov/genetics/condition/congenital-sucras... "After ingestion of sucrose or maltose, an affected child will typically experience stomach cramps, bloating, excess gas production, and diarrhea. These digestive problems can lead to failure to gain weight and grow at the expected rate (failure to thrive) and malnutrition. "

So this is NOT a great mutation. There is no such thing as a universal "great" mutation. These genetic changes are just one change in a probably network of changes in the Inuit. Others would include genes like FADS1 and FADS2 and CPT1A.

(NOTE: I have Saami heritage, who are the Inuit of Finland.)

Next:

"'It is probably due to Greenlanders not having had very much sugar in their diet. For the most part, they have eaten meat and fat from fish, whales, seals and reindeer. A single crowberry may have crept in here and there, but their diet has had minimal sugar content,' "

Here the researchers are correct, this is not only a Greenlander trait, but a common trait in all Inuit, and very low sucrose is common in all Inuit diets. https://pubmed.ncbi.nlm.nih.gov/25452324/

And this absolutely is a fitness example. For people with the common allele, the lack of sucrose in the diet leads to lower levels of acetate and these people would die off leaving the survivors with the less common allele to flourish.