Slack is certainly more well known than its founder, but neither are household names unless you live in silicon valley.
As an example, have you ever watched a movie where someone uses slack? I haven't. Have you ever seen a movie where someone uses google (or some rebranded website that's obviously a google stand-in)? Of course you have. That's what a household name is.
Edit: also if it was rephrased that way the rest of the paragraph wouldn't make sense because they're discussing slack's founder well known-ness, not slack.
As an example, have you ever watched a movie where someone uses slack? I haven't. Have you ever seen a movie where someone uses google (or some rebranded website that's obviously a google stand-in)? Of course you have. That's what a household name is.
Edit: also if it was rephrased that way the rest of the paragraph wouldn't make sense because they're discussing slack's founder well known-ness, not slack.