java.util.concurrent.ScheduledFuture does not implement Comparable<ScheduledFuture>. Instead, it is a subinterface of Delayed, which extends Comparable<Delayed>. In other words, a ScheduledFuture instance isn’t merely comparable to other ScheduledFuture instances; it’s comparable to any Delayed instance, so we have to say Comparable<? super T>. Delayed is super of ScheduledFuture. It seems that the second part, List<? extends T> as a producer does not make any difference.
ScheduledFuture extends Comparable<Delayed>
where Delayed is super of ScheduledFuture