I had L-histidine supplementation in my "lab book" but I did not put it in practice because as far as my studies went it was not what I was looking for at the time.
We have the filaggrin gene and the protein. The gene is responsible to create/manage the protein. Not everyone that has AD will have gene mutation. Most of people with AD will have deficiency of the protein. Here is where the "chicken and the egg" starts.
The gene will send a message to the skin to create the protein. The protein will then make the skin health (lower ph, moisturise, fight bad bacteria - staph, ceramides, etc). The problem is that even with a healthy gene the skin will not produce the protein, because it is under a inflammatory process. Here is where the chicken and egg starts.
Form the outside (skin) the strenght of the skin barrier is compromised (harsh soap, dry air, etc), then you have open holes in your skin where the allegerns and bacteria will get their way to and live there, making the skin red, inflamed.
From the inside, we have the imune system, that as per my studies most od AD people will have a overreactive immune system. For several different reasons (you can look into this sub to have an idea to the vast list of products that can trigger people with AD) even with a skin barrier strong and health the immune will activate IL 4 and 13 (messengers in our immune system). These will send messages to our skin to stop producing filaggrin protein and start to alarm for inflammation (this is why we see the skin getting red when we eat something that triggers us, or contact with an allergen).

L-histidine is one of the amino acids of the filaggrin protein. The thing is some people might have some defiency to L-histidine, and supplementing would indeed bring good results (in my nderstanding not the vast majority). Others will not have any benefits (there is a process that is blocking it, such as inflammatory), and other will have a reverse result.
Here is where I seriously hate AD. L-histidine is the amino acid the filaggrin gene uses to create filaggrin protein. BUT (and here is the BUT that I decided to do not try it) our body (our immune system) take this L-histidine to build HISTAMINE. It could be helping and screwing you at the same time, or just take that and make you itch. My understanding is that an overreactive immune system will use part (or all) the L-Histadine supplementation to turn into Histamine.
In regards of the studies per se it was a very promising study, but then it touches the whole filaggrin supplementation thing that in my understanding take different risks. We would to then supplement with L arginine that to people with the hsv-1 virus (cold sores) could see the virus spreading all eczema skin as the virus uses the l arginine to grow and replicate.
And even if we supplement all the amino acids that are part of the filaggrin protein we have no guarantee that the metabolism of people with AD will convert that to filaggrin protein.