Your initial question (is this really a problem) is completely unrelated to the rest of your post. It doesn't matter if third party sellers are important to Amazon. Nobody cares but Amazon. If 3P are important, they have to do the work to make their marketplace viable.

The problem is that they comingle authentic products from brands with fakes from 3ps. You never know which it is you are buying.

Comingling increases sales on their platform, but encourages fraud. They need to stop comingling but won't until they are shut down by regulators because it decreases their turnover.