The source code for the various components that make up Open Distro is already freely available under an Apache 2.0 license. This change will have zero direct impact on Open Distro. The SSPL restrictions apply when the licensed software is used to provide a service.
It is the AWS Elasticsearch Service that will be directly impacted. It will be limited to Elasticsearch 7.10.x as a foundation. Unless of course AWS makes available the code that they use to orchestrate and manage that service. Assuming that that code is sufficiently uncoupled from other systems, they could perhaps do exactly that. It would certainly be an entertaining counter-move from AWS.