I don't see anything in Russ Cox's answer that proves your statement. If there is any conclusion to that post, it is 'We can probably do it. But we are not doing it now because we are too afraid that we do it wrong, like Java.'
My problem with the Go team's line of reasoning is that they always cite Java (erasure gives problems) and C++ (too long compile times), but there are many languages that have implemented parametric polymorphism nicely ages ago (C#, ML, Haskell).
Sometimes I wonder if it is the lack of a person like Philip Wadler on the Go team who can hash out how to best fit generics into Go's type system.
More like they can't add generics without breaking the backward compatibility of their mediocre type system.