Furthermore, section 13.2 explicitly mentions the exact case we've been discussing (in rather simple language) :
Joined objects are used as a tool for precise specification technique in this standard. They are not meant to be used as a
guideline to how Function objects are implemented in practice. Rather, in practice an implementation may detect when the
differences in the [[Scope]] properties of two or more joined Function objects are not externally observable and in those cases
reuse the same Function object rather than making a set of joined Function objects. This is a legal optimisation because this
standard only specifies observable behaviour of ECMAScript programs.
Step 1 allows an implementation to optimise the common case of a function A that has a nested function B where B is not
dependent on A. In this case the implementation is allowed to reuse the same object for B instead of creating a new one every
time A is called. Step 13 makes this optimisation optional; an implementation that chooses not to implement it will go to step 2.