"Accompany it" is the key phrase. The source needs to be released at the same time as the binaries.
Section 6 states: " As an exception to the Sections above, you may also compile or link a "work that uses the Library" with the Library to produce a work containing portions of the Library, and distribute that work under terms of your choice, provided that the terms permit modification of the work for the customer's own use and reverse engineering for debugging such modifications.
You must give prominent notice with each copy of the work that the Library is used in it and that the Library and its use are covered by this License. You must supply a copy of this License. If the work during execution displays copyright notices, you must include the copyright notice for the Library among them, as well as a reference directing the user to the copy of this License. Also, you must do one of these things:"
And 6B specifically says: "b) Accompany the work with a written offer, valid for at least three years, to give the same user the materials specified in Subsection 6a, above, for a charge no more than the cost of performing this distribution."
So, since Apple is not distributing just the library binary on its own, but rather as part of Safari, Section 6 excepts Section 4. And all they must do, according to 6B, is accompany the work with a written offer (which it does, see Settings/General/Legal) to provide the source. Since no one has specified that they've written to Apple and been denied, they are not in violation of the license.
In the case of Apple, they have made modifications to the library itself. They are then using the library, with modification, in Safari. Safari can be released under any license as outlined in section 5/6 but not the modifications. Those need to be released.