I don’t think that’s the issue. As said in the article, the researchers found the flaw by reading the architecture documentation. So the flaw is in the design of the API the operating system uses to configure the CPU and related resources. This API is public (though not open source) as to allow operating system vendors to do their job. It usually comes with examples and pseudo code on how some operations work. Here is an example (PDF).

Knowing how this feature is actually implemented in hardware (if the hardware was open source) would not have helped much. I would argue you are one level too low to properly understand the consequences of the implementation.

By the vague description in the article it actually looks like a meltdown or specter like issue where some code gets executed with the inappropriate privileges. Such issues are inherent to complex designs and no amount of open-source will save you there. We need a cultural and maybe a paradigm shift on how we design CPU to fully address those issues.