-core predates steampunk as a term by decades. -Core was generally only used when describing musical genre mixing in an attempt to clarify the roots of a particular group’s sound.
The only -punk terms in use prior to the 2000’s were cyberpunk, crust punk, and punk all of which were used to indicate a level of rebellion. Punk is being used in a similar way -core was until steampunk rose in popularity followed immediately by dieselpunk and atompunk cementing the concept of [powersource]-aesthetic as the primary defining trait of a fantasy genre which easily found it’s way into use as a descriptor for an aesthetic that would be expected within that fantasy setting. Things get confused again with the more recent solarpunk (follows the format) and cottagecore (does not follow the format because it is not a musically defined aesthetic)
It’s a pretty classic case of a newer generation believing they’ve invented something without realizing they’ve actually misunderstood prior usage due to limiting their sphere of influences to their peergroup. These are the same types of people who would call people posers for not conforming to the punk aesthetic because they never understood what punk actually was beyond a vector to fit into a group (and all the irony that entails in the context of punk)