On-the-fly grammer redefintion should not affect the actual way in which the type system works. It might play some tricks on the programmer so that a Str type annotation is turned into a Num type annotation, but while that would be kind of silly, it would not affect the way the underlying type system works.

Actually, Perl 6 will try it's best to be as statically typed as possible, including performing some type inference when there are no programmer written type annotations present. Anything which it cannot figure out during the compile phase, it will leave until runtime (which is fairly common in other "staticly typed" languages like Java, etc.). This combination of compile time static type analysis and runtime type constraint checks is sometimes referred to as a hybrid type system, but once again it is not unique to Perl 6 by any means.

However, this may or may not be what they mean. Perl 6 will also combine a nominal type system (types are compared by names) and a structural type system (types are compared by their defined structure). This could be seen as a hybrid type system too, however this too is not unique to Perl 6 either.

So possibly they mean that Perl 6 is unique becuase it's type system is; as static as it can be and as dynamic as it needs to be, will check types by name if it can or by structure if it needs to, and in the end is entirely optional.