Many authors write parallel works, so it is not impossible to in fact have more than one work ready for publishing at the same time. It even makes more sense if we consider that different books will take different amounts of time to proofread, review, etc.
What I do like is, because of this, publishing as it exists can be shooting itself on the foot.
If it becomes accepted that an independent author requires real time to write a book, then the same is true for a publisher backed one. So, if an author can churn out more than one work trough that channel than the indie one, that may imply the publisher is in fact using shadow writers to put more books out than humanly feasable.
There are incredibly talented and focused author that will put out a lot of work in a short time window but that is not the rule for the average person.