To be clear, these three of your "alternatives":
> Most individual sellers are not actually "individual": Same owner or network
> Subscribed to update packages
> Generator to fill forms
...are fully in line with my argument. My hypothesis wasn't specifically that this was a SaaS system doing this. Rather, my hypothesis was that there's a Single-Point-of-Failure entity or platform that these listings go through — a Borg Queen that could be taken down; and that doing so would stop thousands of sellers in their tracks. I'm agnostic to what form that entity or platform takes.
You might be interested to hear my own guess as to a possible mechanism for this, though, as it's not listed among your alternatives. That guess is that there are a very small number of Chinese companies that advertise their services as writing/managing English-language Amazon product listings, for companies that have no English speakers. These companies use human labor, not automation, but they have a strict style guide, which both informs the format of their output, and the type of input they require from their clients. The network of sellers whose listings look the same, are all the work of one such English-language post-localization company (the largest/most popular one), and so all adhere to one uniform style guide. This company, at least, has the seller themselves register with Amazon; but from there, takes over responsibility for creating products in their account, managing returns claims, etc. They promise to "take care of" every interaction with Amazon FBA that requires English knowledge, and to only bother the seller for things that are really important.
This would explain the reluctance of these sellers to engage through Amazon customer service (instead sending cards with their products that say "please report any issues to ") — they don't manage their own product listings (rather, the contractor does); and they don't trust the English-language-product-listings contractor to know enough about their product to do customer service; and it's hard to coordinate their separate English-language customer-support contractor (the one you reach via the email on the card) to be able to receive + respond to messages on postings managed by the product-listings contractor.
It would also make an interesting prediction: that you'll only see the particular style I described in my top post, in Amazon's English-language product listings, because each contract company would likely focus on selling product-listing localization services for a particular language, so different language ⇒ different company ⇒ different style guide.
> e.g. New gossip of the day: "Amazon is going to ban any seller using the characters 【】"
You'd expect some people to miss that news. And even for people who see the update — no matter how "hungry" they are — you'd expect some sellers to let the news slip past them. For every such disseminated "Amazon Seller pro tip", you'd expect less than perfect 100% engagement. And yet engagement with these changes is 100% — at least within this network of sellers where engagement has historically been 100%.
(Just to beat this point to death, consider the ultimate in centralized top-down "do it exactly one way" skill dissemination: driving. Does every 16-year-old who is highly motivated and hungry to get out on the road, learn every rule of the road + constantly execute their learned driving skills perfectly? Sadly, no. Humans are not good at perfectly absorbing skills, and are also fallible at executing them.)