“Open core” is when part of the product is open-source and part is private.
Was a significant part of the product private before this announcement?
If not, someone can fork the repo and immediately launch a competitor (FOSS or paid). (Technically even if so, except it wouldn’t be immediate, and if they’d have to re-implement too much, it would be easier to start from scratch.)
yes, there were a large number of AWS products and features that were only available with a subscription
Yes there were significant portions that were proprietary before this, including support for some services.
The parts that were open source might still be worth forking, but you would probably need to change every occurrence of the name to avoid trademark issues.