Generally, high-histamine foods are not a problem for everybody. However, they can potentially be an issue for those with histamine intolerance. This program isn't tailored to a low histamine diet and it's a very introductory program, so people who react to histamines might be utilizing more coconut milk, more avocado, and that might make them feel worse.
My approach with histamine intolerance is focusing on what's at the root cause. Just like with oxalate issues, I've found that there are things that drive histamine issues. One of them is vitamin B6 deficiency; utilizing P5P can be very helpful. Using some enzymes, like the DAO enzyme, can be helpful as you're working out the root causes. And then I also recommend working on gut health. Some drivers of histamines that I've seen are H. pylori infections, Blastocystis hominis, and mold toxicity. All of them are potential Hashimoto's triggers as well as drivers of histamine intolerance.
Those with thyroid imbalances, including Hashimoto’s, may be more susceptible to histamine intolerance. However, if you know that you are not histamine intolerant, then you will be fine following the program as is.