We all knew that and no one cared.
My thinking was that we now need very high resolution thinking about what types mean, for example, "do they come from functions which have these constraints as output", but that's just as ... well, it's unrealistic. How do you annotate the world?
If your output range was obvious to the compiler, that was never a problem to anyone.
"Type inference is conservative — it may fail to find suitable assignment of type-arguments to the type-parameters despite that one exist, and the function invocation could be successfully typechecked if the programmer explicitly provided suitable type-arguments."
My guess is that what the thesis presents is complete type inference in presence of subtyping that is decidable, efficient, and that yields types with all the nice properties one could hope for (principality, compactness, ...).
Which is hard.
According to Wikipedia, Kotlin first appeared in 2011. Algorithm W for inferring types in the Hindley-Milner type system was published in 1982.