Earlier quoted context omitted.
The web-based implementation means you only have one codebase for both Slack in the browser and Slack in Electron. So adding Qt in addition to their browser client means they have two disparate clients instead of one client (plus Electron glue code).
So what? You're saying that like it's this terrible thing. It's not. Especially because, as has been shown, Electron apps are extremely sorely lacking in things like accessibility.
There are a couple of cases where you can avoid that in the text editor and source control space: Sublime Text is cross-platform and native, and so is SourceTree (although it's become quite buggy). So just avoid GitKraken, GitHub Desktop, VS Code, Atom.