It's extendable and "modifiable". Being written in javascript, you can do all sorts of hacks around the code while not even modifying the source code itself, but using the APIs/other means around it for customization. JS injection, monkey patching, es6 Proxy, etc. Probably doesn't apply with that specific one, but putting a custom CSS onto something also qualifies to be "hackable". Basically all the web/webview based apps can be marketed like that if they don't specifically restrict such thing.