If you sniff the network traffic you’ll find why: it essentially is a hybrid html and native app. All pages of the App Store are set up in XML and loaded server side, hardly ever cached. Just like <select> in HTML iTunes uses similar structure to instruct and deliver native or semi-native components.
It’s a big web page. And it’s slower than Safari. That’s why.
I remember reading speculation about that as well. After some googling, it appears only speculation that the Music app would be built using Marzipan/Catalyst.
9to5mac writes in a correcting update that “We’ve now learned that the code for the new Music app for macOS will be based on iTunes, not iOS”.
I imagine that they didn’t actually use electron, but the app feels pretty far from native imo