[Veratyr]

Slack is chat, its messages are sent and read by humans. Ably is a developer API, its messages are sent and read by machines.

Slack has a shiny user interface and a bunch of nice enterprise features. It's something you'd have everyone in your company sitting on their machine all day.

Ably is something nobody will ever see. It's an API that underlies other people's applications. Hell, Slack could run on top of Ably.

They're absolutely not competing. You're either misreading or deliberately misleading.