I'm assuming Wong 2021 has nothing to say about this other than the assertion.
A basic prerequisite for this would be that ordinary people's genitals are visible through their pants.
>While we did not find any group difference in the experience of playing with toys without genitals in childhood, the frequency of playing with such toys is associated with erotic attraction to castration for the Interested and Wannabe groups. These findings suggest that childhood play with male toy figurines lacking genitalia might potentially contribute to BIID, which for some individuals may fulminate as extreme castration ideations and ultimately genital ablation. Such an impact from toy play resembles other study findings (Fernandez & Pritchard, 2012; Galioto & Crowther, 2013) that exposure to slender and muscular models may lower body satisfaction and motivate body modification for males, in parallel with the Barbie doll syndrome for girls (Dittmar et al., 2006). Yet it should also be noted that there was a higher proportion (59% vs. 45%) of individuals in the Castrated group, who never played with male figurines, than in the Wannabe group. These results indicate that play with male genital-less toys may be significant for some, but not all, in contributing to extreme castration ideations.
https://doi.org/10.1007/s10508-020-01808-6
> We designed a survey to compare the psychological, sexual, and childhood experiences for subgroups of men with extreme interest in castration. The survey was posted on the Eunuch Archive, an online platform for individuals interested in the topic. Data from 822 individuals were included in the analyses.
IIRC, Ken Dolls had tan lines or “permanent” briefs, but could remove pants from many action figures and dolls (and put on skirts and dresses on the males, interchangeably)