Author here: This is one reason why I used that term. I was definitely thinking in terms of the 'phase accumulator' looking up the sin table.

Everyone is absolutely right that this is the same as frequency. I think I was used that particular poetic flourish to emphasise how complicated the academic, mathematic explanation is, in contrast with the diagram. Quite likely I picked up that exact phrasing of "phase angle" from some academic literature in particular.