Scary, and certainly doesn't reflect well on Slack. But, do keep in mind that the author runs a company that does compete with Slack in some ways.
Poor security practices are poor security practices despite conflicts of interests, and Slack's are certainly extremely poor.
They have a lot of high quality security features and you can see they actually work because they alerted Max that his account was compromised.
Saying their security practices are extremely poor based on an incident they had in 2015 when their company was 1/20th the size it is today is ridiculous
Regardless of the issue, that still reflects very poorly on him.