To repeat DiapDealer's point:
Sigil is supposed to produce (and check) valid ePubs, from the point of view of the specs.
If a distributor/publisher/whatever decides to require or prohibit additional features, even if it's called "Apple", then that's their problem (and the problem of whoever chooses to deal with them).
That doesn't mean Sigil (or any other program) is perfect, it has bugs and limitations. If you see any particular fault with the validator, (as in accepting an ePub that doesn't conform with the spec, or flagging one that does), please report it.