The basic idea is, if somebody last committed more than 28 days ago, it is likely that they are not a "core contributor" since you would expect "core contributors" to commit more frequently than every 28 days. Hope that makes sense.
For Next.js, the percentage of people that last committed more than 28 days ago accounts for 80% of all unique contributors, which is what makes their project quite staggering from a "popularity" standpoint. Assuming everybody that last committed more than 28 days ago are not "core contributors" (which may not be accurate of course) we can see that Next.js had 169 contributors that basically decided to create a pull request that was good enough to be merged. For Redwood they had 23, which is respectable but not on the same level as Next.js
Based on my analysis of very popular open source projects, most projects typically only have 50% of their contributors in the +28 days range, while Next.js has 80%. Looking at last commits more than 28 days ago is a very good indicator of community health/interest.
