The article notes that today's youth is more content than earlier generations, so the comparison pertinent to the headline of the article is through time within Japan.

Assuming culture, phrasing, language, etc remain constant.

I know today's Japanese youth and young adults are not very open or honest with their emotions, especially with respect to sharing negative feelings. How that compares to the past, I don't know. It could very well be the same, but take the article's claims with a grain of salt. I just know that I've personally met an unfortunate number of Japanese who are depressed and disaffected.

>Japanese are notoriously not honest with their emotions

Internet stereotypes aside, I'd love a source on that

I don't know what "Internet stereotype" is supposed to imply, but you won't live in Japan long before some Japanese friend takes you aside and warns you about the phenomenon of tatemae vs. honne, a Japanese person's public expression of feelings vs his true, carefully hidden feelings. If you learn to read Japanese, you'll notice whole shelves in the big Japanese bookstores devoted to the analysis (mostly by Japanese authors) of differences between Japanese culture and others, such as why the tatemae/honne distinction, present in all cultures, is so extreme in Japan.

Decades ago, anthropologist Ruth Benedict discussed this issue (among others) in her book, "The Chrysanthemum and the Sword", which was held in high esteem by the Japanese post-war generation. You might start with that.

I appreciate the readings and will definitely check it out. I've noticed the fetishization of Japanese culture (positive and negative) on the Internet so I'm immediately suspicious of any broad generalizations I see of Japanese people.

I wouldn't just call it an internet stereotype. Have you tried investigating this yourself? It should not be hard to find articles, Youtubers, and bloggers discussing this. Or have you lived in Japan, or have many Japanese friends?