Would be better worded as "Slack, co-founded by Stewart Butterfield, might be a household name today, but in 2001[...]"
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.
I guess I don't really watch many movies that delve into 21st-century office communication, but I imagine if I did their chat app would be modeled after Slack.
Overall, though, this is a minor point compared to the rest of the article.
People know who Jeff Bezos is, people don't know Tim Cook.
About a decade later I worked on the JavaScript implementation of NemID, NemID being a security thing in Denmark that basically every citizen has to use.
A coworker said to me (paraphrasing): "It's great to work on NemID, because everyone knows what it is, so it's easy to explain at family dinners" which is true. very true, everyone knows in Denmark what it is.
To which I thoughtlessly replied "Oh yes, it was the same when I worked on eFaktura", his face got the momentarily shocked and dismayed expression people get when they realize they are talking to a crazy person or an idiot.
About 5 minutes later I thought to myself "what the hell are you thinking, nobody knows what eFaktura is, fool"
I think the household name status of Slack or the guy who co-founded it is sort of like the household name status of eFaktura in Denmark.