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Seems like there might be more to it than that, since Slack is written in Electron and is in the Windows Store.

There is something more to it: Slack is really popular and MS really wanted it on the store so they let them ignore the rule.

Slack isn't the only Electron app in the MS Store. There's an entire guide on building Electron apps for the Store:

Microsoft devs have even contributed to the tool mentioned in that article (and Microsoft is listed in the MIT license copyright statement):

The "more to it" is covered in the article.

They don't want things 'browsing the web' without you being forced to use MS Edge.