"CRITIC: According to the principle of relativity the whole affair should proceed in the same way if it is represented in a coordinate system K', that is co-moving with clock U2. Then relative to K' it is clock U1 that is moving to and fro, with clock U2 remaining at rest. It then follows that at the end U1 should run behind U2, in contradiction with the above result. Surely even the most devoted followers of the theory will not assert that in the case of two clocks that have been positioned side by side, each one is running behind the other. RELATIVIST: Your last assertion is of course undisputable. However, the reason that that line of argument as a whole is untenable is that according to the special theory of relativity the coordinate systems K and K' are by no means equivalent systems. Indeed this theory asserts only the equivalence of all Galilean (unaccelerated) coordinate systems, that is, coordinate systems relative to which sufficiently isolated, material points move in straight lines and uniformly. K is such a coordinate system, but not the system K', that is accelerated from time to time...A homogenous gravitational field appears, that is directed towards the positive x-axis...According to the general theory of relativity, a clock will go faster the higher the gravitational potential of the location where it is located, and during partial process 3 U2 happens to be located at a higher gravitational potential than U1. The calculation shows that this speeding ahead constitutes exactly twice as much as the lagging behind during the partial processes 2 and 4." http://sciliterature.50webs.com/Dialog.htm

Clearly, without the acceleration (homogeneous-gravitational-field) crutch, Einstein's 1918 paper is in fact reductio ad absurdum:

An axiom (Einstein's constant-speed-of-light postulate) entails an absurdity (either clock is running behind the other) and should be rejected as false.

Einsteinians know that Einstein's 1918 paper effectively disproves special relativity, never mention it and even repudiate its main thesis from time to time:

Don Lincoln: "The Mistaken Theory on Acceleration. The common response to this paradox, even from physicists who don’t work with relativity would be that the two twins are different as one of them experiences acceleration. According to this theory, Gabby experiences acceleration to catch up the speed, decelerates to turn around, and then again decelerates to land on Earth. So, if acceleration is the answer, it means that while the spaceship is freewheeling between the stars, both twins age equally and when the acceleration turns on, there is instant aging. However, there is only one problem, this theory is incorrect. So, to resolve this, we shall presume that there are three observers: Abby, Gabby, and Tabby. As previously assumed, Abby is stationary on earth and Gabby is heading at a speed of 99.9% the speed of light to the star, Tau Ceti. Tabby, heading toward Earth at the same speed as Gabby, is 24 light years away and along the line of sight between the two stars. It is further assumed that the information on acceleration is not available and insignificant. The time Abby will experience if she were to sit on Earth and watch the clock would be 24 light years or T_Abby. As for Gabby, if she zeros her stopwatch as she passes Abby, the amount of time the clock will display when she reaches Tau Ceti would be half of T_Abby divided by gamma. This is because the total travel time is 24 years, so half the time to reach the star would be 12 years. Gamma is 22.4 and therefore, Gabby experiences a mere six and half months. Now the third person, Tabby heading toward Earth, arrives at Tau Ceti exactly the same time Gabby arrives. As Tabby heads toward Earth, she clears her clock and calculates the time duration to reach Earth from Tau Ceti. Abby finds that both Gabby and Tabby experienced only about half a year in each of their legs, which meant the total time experienced by the travelers was just about a year when Abby waited for 24 years, proving that acceleration had no role to play at all." https://www.wondriumdaily.com/is-the-twin-paradox-of-special-relativity-really-a-paradox/