My AMS campaign just got approved (targeted by product). I know that AMS only allows editing the CPC bid and budget after approval. However, when I clicked on the Targeting tab for my campaign, there was a button to "Add Targeted Products". I tried adding a couple more books. For a moment, it appeared that my previously targeted books were wiped out (!); however, after refreshing the page, the new books were indeed added to the targeting list.
You're right. The help page make it seem otherwise: https://kdp.amazon.com/help?topicId=A2DJUTY13KIH2C. Well, it's correct about interest targeting. And it's technically correct about product targeting; it just doesn't clarify that you can "add" to your target list.