As per my understanding, currently, Core EIPs follow a bit different process than what is proposed in the strawman criteria for advancing EIP statuses.
Generally speaking, Core EIPs are tightly coupled with the Network Upgrade process.
Thus, instead of following “reference implementation” and “number of days” it passes through different phases inviting “rough consensus”and “client’ implementation”.
Here is a high level diagram of status change criteria. Also, refer to the ECH blogpost explaining phases requirements.
As @anett mentioned, Core EIPs in an Executable Spec World is proposed to bring some changes to the existing process of Core EIPs documentation & implementation. Worth following the discussion and sharing thoughts, if any.