That assumes it is acceptable to talk about it equally across genders.

But it's not exactly implausible that people socially gendered as boys face stronger social pressure against revealing insecurities, in general, both in their peer groups and in general.

Even with no difference specific to gender dysphoria, this could prevent the same phenomenon from manifesting with boys, or at least create a higher threshold of societal awareness and acceptance of gender dysphoria before that acceptance combined with the degree of general acceptability of boys revealing insecurities produces a similar effect to what has been seen in girls in this case.

I would say it is far less acceptable for boys to talk about their gender dysphoria than is it for girls, if only because even if a girl wears boys clothes and doesn't 'pass' they'll receive much less attention than the opposite situation.

That's entirely without considering the stigma society imposes on men who want to talk about their mental health at all.

Not necessarily. The underlying cause could be biased towards one gender. We don't know what causes it.

I said "If that is the only explanation...".