There are a few clarifications I would like to make:

Having an erection does not require the person to have sex, and is not the same as being horny or desiring sex.

Having a penis does not guarantee you have erections (let alone involuntary erections, which is what you seem to be talking about). People with penises who are testosterone dominant do have involuntary erections, but even so, see my first point for why that's not relevant.

Your claim was about being horny being a bigger problem for people with penises, which is a fair assumption but has more to do with testosterone than the penis (like you've pointed out, trans men can be very horny without a "penis" - though it should be noted here that male and female genitalia are more similar than dissimilar and have the same structures of a phallus and glans, just in different configurations).

so it’s already suggesting that it’s not a myth.

The myth is the belief that men are horny while women are not, the reality is that it varies significantly by person. with significant overlap between the sexes. There is a difference on average, but it's not as large or total as people commonly believe.

And finally, as you have pointed out the social context will skew the data significantly with fewer women being comfortable with sex than men, fewer women having learned to masturbate than men, and fewer women willing to discuss or disclose their sexual feelings or behavior than men. These differences in how sex is treated socially means whatever biological differences there are is muddled, especially when what was measured was self-reporting on frequency of masturbation. It's possible that men and women are far more similar than dissimilar than even the current evidence we have points to.