Comment by drinchev

It actually is an electron app

    λ /Applications/Trello.app/Contents ◆ tree -L 2
    .
    ├── Frameworks
    │   ├── Electron\ Framework.framework
    │   ├── Trello\ Helper\ EH.app
    │   ├── Trello\ Helper\ NP.app
    │   └── Trello\ Helper.app
    ├── Info.plist

Tried it. It's a bit laggy and seems not much different than the website itself.