Well according to the abstract it related to gut health affects of the coffee not the caffeine itself.

> "Our findings reveal the microbiome and neurological responses to coffee, as well as their potential long-term benefits for a healthier microbiome. Coffee may modify what microbes do collectively, and what metabolites they use. As the public continues to think about dietary changes for the right digestive balance, coffee has the potential to also be harnessed as a further intervention as part of a healthy balanced diet."

That would explain why it works for decaf too. i.e. it's healthy gut, healthy mind.

I did some searching as well. This (unrelated to today's article) seems to say the ritual does more than just ingesting the substance:

https://www.medicalnewstoday.com/articles/drinking-morning-c...