Wish there were some data available on the efficacy of it for them. As a marketer, broadcast is always an interesting one to watch as budgets shift to digital.
Based on the commercial contents, they're targeting pretty much any white-collar employees. So the target population isn't as small as it appears at first glance.
Their operating model also leads to a pretty unique way of internal adoption. At my last company (a smaller one), we had a paid Slack account and it was our official means of internal communication.
At my current company, a large Fortune 100 company, email is still the predominant form of internal communications. With Skype and Teams available, but with a frustrating enough user experience that everyone falls back to email. But there are a lot of pockets that unofficially use Slack, with some using the paid form (and expensing it under their budget) and others using free ones. Our central IT is finally giving in looking at getting an official subscription to centralize usage under a single Slack account (for auditability, information security, employee attrition concerns, and ease of user support).
By targeting end users directly in those commercials and enabling internal adoption without interfacing with IT (i.e. free accounts, web app for those that can't install desktop apps, etc), they're able to gain adoption and momentum in a guerrilla fashion that snowballs into eventual enterprise contracts.