Indeed it seems weird - search Amazon for "secret diary" and you'll find a whole lot of products aimed almost exclusively at girls.
I think the question is whether it was seemed to "cross over" into "spy gear" - while secret diaries are very much a "girly" toy category, spy gear is very much focused at boys.
In 2016, Mattel/Barbie ran (or still runs?) a "Barbie Spy Squad" series of toys and merchandising complete with movie and McDonald's promos, so while rather late (and obv better than never), the toy manufacturers are at least aware of the market segment.
I think the question is whether it was seemed to "cross over" into "spy gear" - while secret diaries are very much a "girly" toy category, spy gear is very much focused at boys.