A big thing about Kakeru saying he's a boy all the time is that it could be denial, as a Trans person myself, I spent my first 16/17 years completely oblivious, then I joined an Anime club at my school and made a lot of close friends.
From 18-21 I self identified as a crossdresser/trap but still didn't feel right about myself, and then I started doing a LOT of self-reflection and meditation when I had it click that the reason I felt uncomfortable in my body was because it didn't fit how I saw myself in my head. So I started HRT the year I turned 25 and haven't been happier. I still have my issues, quirks and qualms, but I actually feel more or less in-line with myself.
Kakeru (and plenty of other characters from other manga I've speculated about in other threads) could just be in a denial stage. Granted, the journey of self discovery doesn't always make you different, it just helps one know who they truly are on the inside.
Whether the author actually has Kakeru stay a boy, or has Kakeru stay a girl, what'll matters the most imo is;
If Kakeru ends up happy or not.
TL;DR:
Just because someone says something doesn't mean that they mean it deep down.
Also, "Still cis tho haha 😅" is a long standing Trans community in-joke.
Also also, no one is wrong until the author finishes the manga.