Nope.
Convergent evolution and/or Genetic Mutation. Right in that posted comic the doctor told A'lars said he would study
why it happened.
If a deviant gene is (for example) a change in a single or even set of amino-acids from a related Eternal gene then that can happen by chance (much like how viruses evolve). This can happen because Eternals and Deviants have the same common ancestors (so have many of the same genetic starting positions). It can happen even without common ancestors (as in the multi-region theory of the origin of mine. Incorrect or not the idea is based on the fact that it is scientifically possible).
That's how evolution works.
Deviant gene doesn't mean "got it from deviants". It means the same gene (constitution) as the one that makes Deviants look (etc) like they do is present. It does not mean that Thanos is a Deviant or even part Deviant anymore than wings mean an Ostrich and Bat have a direct evolutionary path. Or anymore than virus strain A and B having a common genome subset means that A and B both got it from a common ancestor or that either A or B got it from the other.
"
Convergent evolution is the independent
evolution of similar features in species of different periods or epochs in time. Convergent evolution creates
analogous structures that have similar form or function but were not present in the last common ancestor of those groups. The
cladistic term for the same phenomenon is
homoplasy."
Convergent evolution - Wikipedia
Gene mutation is a random walk down a tree of mutation paths. Independant subsets of such a tree will have common factors that are not present in common forebears. As for example we see this with covid as discussed here
The Coronavirus Variants Don’t Seem to Be Highly Variable So Far . Yeah geneticists use better nomenclature than Marvel for naming mutations than Marvel's use of "Deviant" but that is because one is contemporary 2020 science from qualified scientists and the other is 1960's science based fantasy from non-scientists.
That is not to say that there wasn't a deviant ancestor in Thanos bloodline
but the evidence presented doesn't mean that there was.