I've heard this, but I honestly don't see it in practice.

If you've been to a Costco recently you may be surprised to find that they're much less expensive than Amazon. They have more buying power and stronger vendor relationships that AMZN simply doesn't have. Prime Day 2017 was also a huge disappointment (Mostly overpriced junk like bluetooth speakers).

Honestly I think the only thing that works in Amazon's favor is the convenience, and their private distribution/delivery network. Beyond that they're nothing more than a generic online marketplace.

That's great for consumers.

Not necessarily. It could easily lead to inferior products as every point on the pipeline from manufacturers to distributors cut costs. It can, in addition, lead to a monopoly situation providing for relatively unconstrained pricing.