  In his recent article [arXiv:1604.04950], Adler questions the usefulness of
the bound found in our experimental search for genuine effects of hyper-complex
quantum mechanics [arXiv:1602.01624]. Our experiment was performed using a
black-box (instrumentalist) approach to generalized probabilistic theories;
therefore, it does not assume a priori any particular underlying mechanism.
From that point of view our experimental results do indeed place meaningful
bounds on possible effects of "post-quantum theories", including quaternionic
quantum mechanics. In his article, Adler compares our experiment to
non-relativistic and M\"oller formal scattering theory within quaternionic
quantum mechanics. With a particular set of assumptions, he finds that
quaternionic effects would likely not manifest themselves in general. Although
these assumptions are justified in the non-relativistic case, a proper
calculation for relativistic particles is still missing. Here, we provide a
concrete relativistic example of Klein-Gordon scattering wherein the
quaternionic effects persist. We note that when the Klein-Gordon equation is
formulated using a Hamiltonian formalism it displays a so-called "indefinite
metric", a characteristic feature of relativistic quantum wave equations. In
Adler's example this is directly forbidden by his assumptions, and therefore
our present example is not in contradiction to his work. In complex quantum
mechanics this problem of an indefinite metric is solved in second
quantization. Unfortunately, there is no known algorithm for canonical field
quantization in quaternionic quantum mechanics.
