I am not very familiar with both integration of vectors and Kepler's equation.
(Don't be surprised. I am still in my Pre-U studies and will only enter university September/October next year
The good news is I will be studying for a degree in Physics. :lol
I think the text is right. (Do you mind sharing the link with me? Thanks)
After all, when you derive the formula for 2-body problem, it is the polar equation of conic sections that contains the magnitude of the radius vector, r and the true anomaly, [MATH]\theta[/MATH].
[MATH]r = \frac{a(1-e^2)}{1-e\cos\theta}[/MATH]
So, if you treat [MATH]r[/math] as the radius vector, I think you should still be able to integrate a vector wrt to a scalar. (I think you are good at that.)
That said, if you integrate the
magnitude of the radius vector, you should get a constant straight line graph for mag(radius vector) against time graph. Hence, integrating mag(radius vector) wrt time should give you a linear graph with no y-intercept.
I don't see any significant physical interpretation in the absement for satellite motions though. If it were significant, I am pretty sure we would have come across it in Pre-U/undergraduate courses.