When you put it that way, I wonder if maybe the parts of the brain that are getting activated are related to manipulating objects with the hands? Like, not to be TMI, but for me a big part of eroticism is wanting to touch people in a kind of stimmy way, like e.g. a big part of the erotic appeal of breasts for me is "those look like they'd be fun to play with!" This might be an autism thing, it does feel like an eroticization of stimming, but that sort of playful touching happens a lot in normal sex, so I think it's a pretty significant part of neurotypical sexuality too. It seems pretty plausible that this sort of behavior might involve a lot of the same neural pathways that get used when you're opening a can or using a wrench or something like that. Especially because this sort of behavior is usually done with an intentionality of wanting to make the other person feel good, so it's not just that you're touching stuff but also that you're doing it to achieve specific results and you're modelling the properties of what you're interacting with to figure out how to achieve certain results with your manipulations.
And thinking about it I think you also see interface-with-a-tool-like dynamics in the ways people use other body parts during sex too. Like, giving somebody oral sex has the same sort of dynamic of trying to physically manipulate parts of the other person to achieve a result, and the same would apply to using your mouth to more generally stimulate your partner through kissing them and so on. And even with PIV sex, it seems pretty common for men to conceptually approach it as kind of like using a sex toy on their partner; I don't think it's an accident that one of the euphemisms for penis is tool. I know I do this!
A cliche for saying that somebody is good at sex is saying they play their partner's body like a musical instrument!
I mean, it makes sense if you think about how incredibly important and ubiquitous manipulation of tools and manipulation with the hands is for humans; it makes sense that those neural pathways would get used over and over, for lots of different stuff! Like, remember those sensory homonculi illustrations that show a human as some weird creature with a tiny body and giant head and hands, using that to illustrate how prominent different parts of your body are in your tactile world?
The one thing (aside from it being pure speculation) that makes this hypothesis seem kind of shaky to me is... Did they check if this happens with women too? I feel like it totally would check out if it did, given the "hand pics are the lesbian equivalent of dick pics" thing, but it seems basic charitability to assume if this is being talked about as a male-specific thing there actually is a pretty noticeable gender difference in it (although actually, now that I re-read your post, you didn't actually say this was being mentioned as a distinctly male thing). If there is, one possible explanation I can think of is... this is making me remember a post I saw here a while back that suggested that an implicit message of the cinematic language of porn is that sex is basically for women; the way that person put it was something like "yeah, the Large Penis is there and there is some indications that what's happening feels good for the dude, but it's basically the woman's expressions of pleasure that are getting most of the attention." It does seem plausible to me that male sexuality tends to involve conceptualizing one's own body as something tool-like and approaching sex as a kind of skilled mechanical labor more than female sexuality does; narratives of a man being a good lover put a lot of emphasis on performance, and per "auto[X]philia is actually a very common sexual desire/behavior that lots of cis people do," autoandrophilia often seems to work that way. I think this is the male equivalent of what that "women often have an easier time connecting to their desire to be desired than to straightforward sexual desire" thing is for women.
I.e. my hypothesis in quick summary: the part of the brain that lights up when looking at half-naked women is the same part that lights up when looking at tools, it's not cause you're seeing the woman as tool-like, it's cause the possibility of having sex registers as a "I might be about to do manual manipulations and use tools" situation.