Because the context here is the idea of using the type system to justify removing those sorts of dynamic checks to generate better code.

The dynamic checks in the Java case are are a well-defined and narrowly-targeted part of the language semantics- you get an exception on mismatched array writes, out-of-bounds access, etc., but when an expression produces a value it always matches its type.

TypeScript defers these kinds of type system violations to the underlying JavaScript engine, which makes things work out (sometimes with an exception, but sometimes just proceeding with a value that doesn't match the expression's type) using precisely the dynamism we wanted to get rid of. And this can leak out and cause arbitrarily-far-away parts of the program not to match their types, either.