"A security incident" is a nicer way of saying "security breach" once you run it through legal counsel.
The article you're reading states...
"We took comprehensive steps to _contain_ and eradicate unauthorized access"
That's a breach my friend.
That's a mixpanel breach if the unauthorised access was mixpanel staff accounts.
If someone phishes your gmail account, there is no gmail breach.
That's a mixpanel breach if the unauthorised access was mixpanel staff accounts.
If someone phishes your gmail account, there is no gmail breach.