Maybe, but... Twitter actually has a mobile website. You can argue - and I would agree - that "request desktop site" should give you what you asked for, but there's still a difference between "here is the mobile web UI" and "here is a link to download an app": namely, one still allows you to use the site if you can't or won't install a native app.

Slack is actively locking folks out of their site here, a service that folks have paid for even... That is a significant departure from what folks might've seen in the recent past with mobile device detection.