The big three cloud platforms all have this issue of delayed status updates. Why do you think it's just GCP?
The status page will always lag the outage. It's not a conspiracy.
20 minutes seems pretty reasonable to me.
Now, I'm on Azure, but it seems like from the comments the situations are similar. So, instead of an automatically updated status page that would help engineers do their jobs, we get a status page that isn't accurate, and customers have pull teeth to get a service credit where/when one is due. And it seems like you can have the cake and eat it too here: while IANAL, a footnote in the SLA or the status page that "this is a machine estimate and not reflective of what goes into the SLA" should do it, no?