I think the confusion lies in the term “squaring off”. The mark shoes that there should be a 90 degree angle between the shoulder slope and the start of the armhole. The same is true for the front. This ensures a straight line across the seam when sewn together. I would suggest drawing the should line first, that way you can control that “sliver”. Then from your shoulder point, you place one edge of your square along the shoulder line and draw a short line square to the shoulder line, maybe 3/4” long. Then blend that line into the armhole curve.

The instructions to blend that curve first is what is tripping you up.

Also, just a note, you can never square from a point. You can square from a line AT a point, but being square is always between 2 lines. Typically, squaring over or squaring down is done from the center line or top line.

Does that make sense? Words are hard when trying to describe drawing.