There's a reason it's called "diligence", because it typically involves
trying. Personally, I don't count reading a company's ad copy on their own web page as being even remotely diligent or doing much research. Normally 'due diligence' involves talking to the party you intend to engage with, as well as talking to others who have experience with that party. The author didn't mention any of that.
Other users here also pointed out that there is a known specific API error message for the user limit. That kind of thing could be uncovered with a little diligence. Author could also have setup a fake team and tried to add many users before putting 8k real users on the team and just hoping.
He did finally start talking directly to Slack only after he started having trouble, and they responded and disclosed the existence of a team size limit. This obviously could have happened before hitting the limit.
The author expressed incredulity at what would be a $500k price tag, and yet completely fails to see why that is a massive glaring red flag. That alone should have been enough signal that his plan was skating on thin ice, but instead he drove right past all the warning signs and then blamed Slack for it.
