There's definitely a legal & contractual difference between (1) storing the books on your servers in order to provide them to end users who have purchased licenses to read them and (2) using that same data for training a model that might be used to create books that compete with the originals. I'm pretty sure that's why GP means by "sucking up."
This is analogous the difference between Gmail using search within your mail content to find messages that you are looking for vs Gmail providing ads inside Gmail based on the content of your email (which they don't do).
Yeah, I guess the "err" is on my side, I've always took "suck up" as a synonym for scraping, not just "using data for stuff".
And yeah, you're most likely right about the first, and the contract writers have with Amazon most certainly anticipates this, and includes both uses in their contract. But! Never published on Amazon, so don't know, but I'm guessing they already have the rights for doing so with what people been uploading these last few years.
This is analogous the difference between Gmail using search within your mail content to find messages that you are looking for vs Gmail providing ads inside Gmail based on the content of your email (which they don't do).