It is not that simple too. Tools used for reasons outside of work may not always be perceived as good tools for work. And tools used professionally are not always considered to be good for use for personal reasons. If someone have met Slack for the first time at their job, they may recommend it at their next job, because they associate it with work - this conversion channel is pretty clear and established. But the conversions work<->personal are not so obvious and may perform worse. I'm pretty sure Slack marketing team controls awareness part of the journey and they understand well their conversion funnel. Ditching some non-converting users is a bold move, but it may be worth it and it may fit well in whatever sales strategy Salesforce has in mind.

it's not that simple either:

tools used for reasons outside of work may be perceived as good tools for work;

and tools used professionally are sometimes considered to be good for use for personal reasons.

tl;dr it doesn't need to be true always, only sometimes, which it is

Well, if I wasn't clear enough: the fact that someone have used a tool for personal reasons and may recommend it as a good tool for work is not enough to justify spending on those personal use cases. You need this to be a healthy conversion funnel and valuable brand awareness channel, otherwise you are wasting money. To make a call you need data from marketing research and I would assume that Slack marketing team knows more about it than HN crowd, simply because they are in position to have this data and HN crowd is not. It can be the case that they are not acting professionally, but I would not assume unprofessional behavior by default.

I respect your opinion of,

someone hav[ing] used a tool for personal reasons and may recommend it as a good tool for work is not enough to justify spending on those personal use cases

I am sharing that I have an equal and opposite opinion, which is that the someone having used a tool for personal reasons and may recommend it as a good tool for work sometimes IS enough to justify spending on those personal use cases

You are right that data can inform the decision, of course, keeping in mind that the benefits include intangibles like goodwill, which goes further in the techie community than in a layman community imo