That article states as fact that Trump couldn’t fire Comey without cause. That is flat wrong. Trump could fire Comey becuase he didn’t like the color of tie he wore on a given day. Trump as President has absolute power over the Executive branch. There are only political prices to pay for firing Comey, as we have seen.
The article attributes that rule to WH lawyer Uttam Dhillon. The article is critical of that rule, and agrees with your take.
Not that it matters, Comey was fired for cause. See Rosenstein memo. Comey usurped the role of the prosecutor in exonerating Hillary, and said he would do it again.
The executive department is at war with its leader.
Now that I think about it, Mr. Trump fired Comey for whitewashing the investigation into democrat corruption. So that was a pretty big swipe at the DS right there, we sort of forget how big.