Nod, you are absolutely right in recognizing the similarity to copyleft. But, this isn’t literally a copyleft software license or CC Share Alike license, it’s taking a step back to apply the fundamental principles of a “strong” form of software freedom in a different technical domain. As technology evolves, the legal tools we use evolve too. The original Open Source Definition never said “copyleft”, it defined the principles in a way that left room for specific legal tools to evolve. For historical perspective, think about how the AGPL was introduced years after the OSD was codified, to address the fact that earlier copyleft licenses hadn’t considered SaaS at all.
The post you linked is talking about the limitations of existing copyleft/Share Alike licenses in the context of AI training, and mentions two possible approaches to resolve the limitations. Other groups are already discussing other possible approaches.