Indeed, calling it Bar<String> seems really weird to me. Usually, in a declaration, the thing inside the angle brackets is a the name of a parameter, which can assume various values at runtime:
class Foo<T>
class Bar<String>
The fact that an instance of A is assignment compatible with a location of type B does not imply that a class of type A is assignment compatible with a location of type class of B.
This is not the case for languages like C#, Java, C++ etc. (despite those languages not having class types); if a subclass does not define all the constructors of its ancestor class, the subclass is not a "drop-in replacement".
Delphi does have class value polymorphism that mirrors instance polymorphism, and it can be a source of confusion, not to mention type holes. For A inheriting from B, you can construct a value of type A using a constructor of B if you assign A to a location of type class of B; A's constructor won't run, and its assumptions and invariants won't necessarily hold. It's one of several problems that makes designing robust classes in Delphi awkward.