Electronic – How does this clamper circuit work

The short-circuit in red puts the 2 points A and B at the same voltage, bypassing the capacitor C2.

This is interesting. Remember that in addition to the equivalent resistance change, the source becomes $$\\V_{thev} = V_{\text{sig}}\frac{R_2}{R_1 + R_2}$$ when you find the Thevenin equivalent. Then, when you perform some calculations using the time constant $$\tau_{thev} = \frac{R_1R_2}{R_1 + R_2}C = \tau_{paper}\frac{R_2}{R_1 + R_2}$$ it is similar to using simply \$V_{\text{sig}}\$ as the source with time constant \$\tau_{paper} = R_1C\$ , since the \$R_2/(R_1 + R_2)\$is common, and also because for \$R_2>>R_1\$, that factor is approximately 1. The scaling of the "equivalent" voltage equals the scaling of the "equivalent" resistance.

\$R_2\$ will limit the voltage across the cap to \$V_{thev}\$, since there will be a steady-state DC current through it. The paper's model is pretty similar, but not exactly the same. I'm not sure what I'm adding, but this can give you an idea of the difference between actual and the paper's model

$$V_C(t)=V_{thev}(1-e^{-t/\tau_{thev}})$$ (with some manipulation) $$V_C(t) =\frac{R_2}{R_1+R_2}V_{sig}(1-e^{-\frac{R_1t}{R_2\tau_{paper}}}e^{-\frac{t}{\tau_{paper}}})$$ The difference is the \$R_2/(R_1+R_2)\$ out front and the \$e^{-\frac{R_1t}{R_2\tau_{paper}}}\$. The approximation is very accurate as both factors approach 1, which happens when \$R_2\$ is large and\or \$R_1\$ is small. Imagine what happens when \$R_1=R_2\$. The factor out front becomes half, but the effective time constant is also cut in half. The result is the approximation seems like it could still be close without the condition \$R_2>>R_1\$, as long as you don't approach steady-state, since the half-as-small steady-state voltage might balance with the half-as-long time constant. But to be sure, we should check the derivative

$$\frac{dV_C}{dt}(t)=\frac{V_{thev}}{\tau_{thev}}e^{-t/\tau_{thev}}=\frac{V_{sig}}{\tau_{paper}}e^{-\frac{t}{\tau_{paper}}}e^{-\frac{R_1t}{R_2\tau_{paper}}}$$ Its value is very similar to the approximation, and notice how the resistor values divide out of the equation, except for the factor of \$e^{-\frac{R_1t}{R_2\tau_{paper}}}\$. Yet despite the first factor dividing out, the function depends on \$R_2>>R_1\$ in order to be a good approximation.