Even in large companies, hiring and firing is not solely in the domain of the HR department. Firing decisions usually involve at least the employee's direct superior. Given that SendGrid is a small company, and that this employee was involved in a very public controversy, I imagine that the executive team also had to weigh in on the matter.

Because the controversy involved an allegation of sexism, a prudent company would also bring in a lawyer. Besides the usual covering-the-bases needed when firing anyone, you have an employee who believes she acted to stop sexual harassment. She could very well try to frame the firing as something that stems from gender discrimination.

Oh yeah, you also want to talk to the employee herself.

So, trying to get what is at least 5 to 6 different people on the same page and in meetings takes time, logistically, to schedule. Nevermind the time it takes to investigate the matter and doublecheck the facts and have give-and-take debates about it.

You really think that when a enterprise company like SendGrid goes under attack, only the ops people are the ones up late at night? Don't you think the executives and customer relations have some weight to pull?

All of the above factors make it difficult to believe that the firing took place after a deliberate, thorough process in the span of two days. It's not impossible, but it's unlikely enough that no one can be blamed for being skeptical or surprised.