Since so many of you thought I was mistaken about the new image naming policy – I reached out to Amazon support for an answer. It turns out you were correct.
Here is Amazon’s answer – Note that images uploaded in Seller central are not affected:
As per the case details, I understand that you want to know if the following instruction applies to all products or not:
File names must consist of the product identifier (Amazon ASIN, 13-digit ISBN, EAN, JAN, or UPC) followed by a period and the appropriate file extension (Example: B000123456.jpg or 0237425673485.tif)
Please be informed that the above instructions are related to the products that are uploaded in Author Central and Vendor Central accounts by Authors and Vendors.
Those instructions are applicable to all products.
However, this is not applicable to products that are uploaded in Seller Central account. For image requirements and instructions for products uploaded in Seller Central account, please refer to the link provided below:
https://sellercentral.amazon.com/gp/help/1881?referral=A2Q7P49OH3AA4U_A3IOTGCM51Y0BB
We appreciate your understanding in this regard.