Over the past twenty some years, we have learned that icy worlds can be much more internally evolved than once believed. Evidence for subsurface oceans has been found at several icy moons. This agrees with data from carbonaceous chondrites showing that alteration processes profoundly affected the chemistry of other volatile-rich bodies from the outer solar system. On the other hand, the default assumption is that large TNOs should exhibit primordial compositions inherited from the solar nebula, as their cold surfaces can support volatile preservation. Here, we report results from recently published papers on the surface compositions (Grundy et al., 2024) and geochemical evolution (Glein et al., 2024) of Eris and Makemake. From JWST NIRSpec data, and have been discovered. These are the first observations of minor isotopologues on large TNOs. is also detected on Eris, but we do not find evidence of on either body. Because observed ratios are not high, they do not indicate the presence of primordial methane like that found at comet 67P. Instead, they show agreement with geochemical models for methane produced in the interior. This inference is also consistent with the lack of detectable on both TNOs. To explain the presence of secondary methane, we suggest that Eris and Makemake hosted (and perhaps, could still host) warm or even hot geochemistry in their rocky cores. Cryovolcanic processes could then deliver methane to the surfaces of these bodies. This delivery may have occurred in geologically recent times to prevent a large enrichment in the ratio (which is not seen) from building up due to atmospheric escape, or active surface processes may be refreshing methane isotopically. We suggest that large TNOs may not be seen as “giant comets”, but they can be evolved worlds shaped by internally driven processes.