They are only required to provide the source upon request. If they are rejecting those requests, that is another matter, but that doesn't seem to be the case here.
The problem is when they take 6 months to respond to such requests. What if they decide to wait a year next time? The ambiguity of "timely manner" in the license isn't helping, perhaps it's time for an improved version of the license.
Doesn't say anything of "timely manner" in the license text. It just says that besides distributing the sources with the binary it's sufficient to accompany "a written offer to give any third party the source code".
Thus, they're on clear only if they give the source code to anyone who asks. Apparently they don't: giving it "soon" is not giving the source code but merely stating a promise to do so which is not allowed by the license. Ditto for not responding.
"You may obtain a complete machine-readable copy of the source code for the LGPL-licensed portions under the terms of LGPL, without charge except for the cost of media, shipping, and handling, upon written request to Apple."
My point is that if it takes a year to "process" such requests, it renders their promise to share the source kind of useless. Now we can argue about how many months would be acceptable, but I'm not sure there's much of a point in that. So a license that has some more specific rules regarding how such requests should be handled, would be an improvement.