Slack is text, channels, images, video, sound, search, audio calls, video calls, screen share (and interface share), bots, myriad integrations, and more. Calling it just "send text from one computer to another" is wrong.
I think maybe their point is that even if other pieces break, why shouldn't it be possible for the text communication to keep working?
If trying to provide all the other things besides text causes the system to be unstable, then maybe those things shouldn't have been added. We need text. We just want the other things.