Greg and Linus are also some of the most visible participants, and as far as I understand, neither is considered particularly expert in the platform intricacies

Greg and Linus are also some of the most visible participants, and as far as I understand, neither is considered particularly expert in the platform intricacies that would be involved in mitigating something like Spectre. Disclosing vulns to high-profile targets is a risky practice; their activity, both public and private, attracts a lot of attention. It makes sense to avoid that risk if possible.