Actually many of the red flags aren't really red flags. I already considered to completely ignore this article. But some, especially 8 and 9 are quite valuable to people who have never experienced that.
The ones I consider serious are these: 1 (hire/fire fast), 4 (given responsibility without authority), 6 (unclear product), 8 (weird money habits), 9 (law problems).
Others I consider quite important: too much money without backing in team strenght and professional attitude (either founders or investors are bullshitting someone), don't trust the team (when shit hits the fan everybody will cover their ass and taking care of others may be considered breaking your employers trust), act context aware (opinion is more important than actual facts, consider when to tell which fact/idea to whom, when everybody is happy because of a successful lie then you have low leverage for the moment).
In any case I think you need to have a few of these experiences to be able to survive in a business environment. When stakes are high, information is hard to come by, and you don't have years to learn to know the other parties, then people will do unexpected things (lying, faking success, spying, or simply not supporting you when you are under attack). Important resumee point: I survived X scams.