Electron apps can be submitted to the Apple store...
Slack has recently been updated for example.
This seems to only affect some developers, especially electron <= 5
5 doesn't work either (others having the same issue in GitHub even got rejected with electron 3). Slack was updated a few days ago (I also had no such an error then). Also let's not forget that Slack is #1 in the App Store for the category business so is certainly not treated like a small app from an unknown developer ;)
It would be a PR disaster for Apple and the MAS to block Slack from updating its Electron app.
Does Slack use private APIs?
No electron application use private APIs, it's the embedded chromium that do.