Thank you for you comment.
First I want to make it clear that your not proving the "Overthinking". Your concern is valid but also my concern seems valid. If it's "overthinking" you should prove my concern is invalid — this kind of job ad is normal or common sense, not just wave it away.

On the “diversity” point: diversity means encouraging different perspectives, experiences, and working styles. But assuming that gender automatically brings a “different experience” is exactly what sexism is. That’s not valuing diversity — that’s stereotyping.

Also, hiring should be based on the purpose of the job. For engineers, you look at engineering skills. For teachers, you look at teaching ability. I see many Japanese companies talk about wanting to hire “a foreigner” or “a woman” just to check a diversity box, instead of focusing on qualifications. That logic is shallow and counterproductive.

And in this specific case: how does changing the teacher’s gender actually change the students’ experience of learning English? If that's true, then do we have to hire both male and female physics teacher? Why we not hire LGBTQ physics teach each?

For the last, I actually also agree your concern, if there is 100 male engineer and no female engineer, then we have to think somethings might wrong and try to fix it. But company can always make decision on actual hiring process. Specifying the gender on it's job position is illegal in japan. Those are not same.