While looking through my English handbook and reading through an example essay, I came across the following sentence:

"Exasperated, he finally started down the winding trail without us and we quickly hoisted our gear to follow him."

I was initially quite shocked to find an absence of a comma between "us" and "and" because (1) my curriculum has always been excessively strict with using proper grammar and (2) it is my grammar book. Then it occurred to me that maybe there's some grammar exception that I have not been taught yet.

Am I correct in the assumption of my ignorance? Or is this an actual grammar error that was made when writing this paragraph?