So you see the equation is NOT distance/velocity as if the velocity of light was c-v and c+v?
The equation comes about because the end point is moving.
It's a general analysis of Sagnac devices.
In some situations, the observer is moving with respect to the device, sometimes not. But even with SR adjustment the formula is essentially the same, and the numerical results the same to several significant digits. So yes, the analysis is valid in both contexts.
And as far as the statement about 2c, it's not really material to the issue if you recognize that using c+v in the expression is not implying that light is going at c+v.
The terms c - v and c + v express the fact that something is going at c - v with respect to something and that something is going at c + v with respect to something.
In the classical view on the experiment, the two light beams are going at c - v and c + v with respect to the observer, while they're going both at c with respect to the aether.
The formula contradicts Special Relativity, since in Special Relativity both beams would have to go at c with respect to the observer as well as go at c with respect to the imagined reference system of the classical aether. In Special Relativity light beams go at c with respect to any and all objects and reference frames.
The magomatician authoring the article on
mathpages.com says:
When it is pointed out that those quantities do not refer to the speed of light, but rather to the sum and difference of the speed of light and the speed of some other object, both with respect to a single inertial coordinate system, which can be as great as 2c according to special relativity [...]
That's an error, since in Special Relativity you cannot simply write c - v or c + v to denote the speed of light with respect to a given object, where c and v are the speeds of light and of the object with respect to the same inertial system. Rather, you have to use a special
Velocity-addition formula (see
Wikipedia). Velocities u and v are added using (v + u) / (1 + (vu)/c^2). Here we have u := c and thus the relativistic sums are:
(v + c) / (1 + (vc)/c^2) = c (v + c) / (c + (vc^2)/c^2) = c (v + c) / (c + v) = c
(v - c) / (1 - (vc)/c^2) = c (v - c) / (c - (vc^2)/c^2) = c (v - c) / (c - v) = c
The result does not surprise, since in Special Relativity the light beams have to move at c with respect to any objects.
Consequently the equation

is correct for the classical case and wrong in the context of Special Relativity, since the used additions of speeds c - v and c + v are classical additions and not relativistic ones. As shown above, relativistically the sum c plus v yields c and the difference c minus v yields c, too. Using relativistic additions, the resulting displacement in fringes Δt is zero, since both beams go at c.
Conclusion: the Sagnac experiment falsifies Special Relativity.

Albert Einstein
Zur Elektrodynamik bewegter Körper. In: Annalen der Physik und Chemie, 1905

Sagnac, Georges:
L’éther lumineux démontré par l’effet du vent relatif d’éther dans un interféromètre en rotation uniforme. In: Comptes Rendus., 1913