I am, I suppose, a gender realist (thinking scientifically, & also pedantically!!), in that I don't believe that a man can truly become a woman (or vice versa). Not in the genetic sense*. To me, that's just common sense.
To me, the closest comparison I can make is that I'm Irish, lived there for the first 20 years of my life - then moved to Scotland & have lived here for nearly 40 years. I'm both Irish & Scottish in a way - the law says I'm Irish, but I vote here & am immersed in my life in Scotland! I hope that doesn't sound flippant (my trans friends don't think so).
The study doesn't say that the brain of someone who's transitioned becomes that of the opposite sex in its characteristics - it says that brain activity shifts to become more aligned with gender identity. That's not the same thing.
However, I also believe very strongly that men who have transitioned should not be considered the same as cis men - they are trans women. I hate the mockery on here of trans women, the insistence on calling them men or men in "womanface", the assumption that every trans woman is a sexual deviant & opportunist. It's loathsome & hateful & bigoted. IMO there are extremists on both sides of the debate & those are the voices that are heard & make headlines.
I am also a trans ally with several trans friends (all MtF), & believe that the change in law regarding toilet access is wrong & humiliating & had not been thought through properly - then or now.
*I'm excluding the rare hermaphroditic/genetic anomalies here for simplicity. That confuses the issue unnecessarily.