The heads were each neatly severed. Because the effects of the drugs hadn't worn off yet, these nobles had genuinely demonstrated the concept of "entertainment leading to death" through their actions.
As for the other prisoners kneeling in the government office, Zheng Fan didn't order them to be beheaded as well.
Although Yan highly valued military achievements and adhered to the method of counting military merits by tallying severed heads, honestly, not many people had actually been killed during this storming of the city.
Compared to the quantifiable military merits from beheadings, returning with the heads of high-ranking officials, led by the Prefect, held far greater symbolic significance.
It would also lend more flair to your boasts back home.
As for how to make such boasts sound somewhat believable, Zheng Fan knew he would have to consult the blind man after he returned.
For instance: "Commander Zheng led four hundred Tiger Guards and obliterated Mianzhou City!"