160F/71C are the general food safety internal cooking temperatures given for most foods. The rationale is achieving an instant 7-log reduction in pathogens - 1 surviving bacteria per 10,000,000 viable bacteria per mass unit of food for the most heat-resistant organism, typically Listeria monocytogenes.
The concern in seitan products is more for ready-made refrigerated commercial varieties, as these may carry pathogens from handling prior to packaging and the seitan itself has a nutrient profile hospitable to their growth.
The difference in time to achieve lethality at 158F vs 160F is negligible without even considering dwell time, and is fine for safety.
Regarding texture - two article abstracts on temperature effects on gluten structure:
[...]Chromatographic examination showed that free sulphydryl groups were
found predominantly in glutenin aggregates of lowest molecular weight
and in gliadins; sulphydryl groups in these glutenin species were
particularly involved in the changes occurring at 55–75°C. These data
indicate that there are heat-induced alterations in gluten proteins at
temperatures above 55°C, which appear to be involved in the loss of
functionality (baking performance) on heating. It is postulated that
the glutenin proteins are unfolded on heating up to 75°C and that this
facilitates sulphydryl/disulphide interchange between exposed groups.
The protein is then ‘locked’ into the denatured state on cooling due
to this disulphide bond rearrangement. At temperatures above 75°C the
gliadin proteins are also affected, involving similar mechanisms.[...]
J.D.Schofield, R.C.Bottomley, M.F.Timms, M.R.Booth, The effect of heat
on wheat gluten and the involvement of sulphydryl-disulphide
interchange reactions, https://doi.org/10.1016/S0733-5210(83)80012-5
[...]Oscillatory measurements of optimally hydrated vital gluten describing
network properties of the material show two structural changes along a
temperature ramp from 25 to 90 °C: at 56–64 °C, the temperature
necessary to trigger structural changes increases with the ratio of
gliadin to total protein mass, determined by reversed-phase high
performance liquid chromatography (RP-HPLC). At a temperature of 79–81
°C, complete protein denaturation occurs.[...]
Monika C.Wehrlia, Tim Kratky, Marina Schopf, Katharina A. Scherf, Thomas Becker, Mario Jekle, Thermally induced gluten modification
observed with rheology and spectroscopies,
https://doi.org/10.1016/j.ijbiomac.2021.01.008
The recipe you provided gave 160F for an internal temperature when baking, which also falls in the ranges stated above. The 190-200F+ temperature for breads is for the rapid dehydration of starches via escaping steam, otherwise the crumb would remain damp and doughy.
The gluten structure in the linked recipe also appeared to have already been well-developed prior to heating. Gluten structure can be formed at room temperature as seen there and in high-hydration no-knead bread recipes.
My experience with canned seitan and fried seitan, both commercially produced, is that it can easily withstand boiling temperatures and higher with little to no effect on texture. The starch component in the chickpeas, however, may go past gelling point and weep moisture when chilled if over hydrated and boiled.
