I don’t see it as missing the point but rather expansion of the point. The roots-as-conservative and branches-as-progressive is a workable metaphor and I don’t see any harm in also mentioning the core.
The danger of progressive-as-branches or height is continuing the metaphor into “pruning”. You don’t usually prune roots but you do prune branches.
If you prune branches, this can lead to a healthier more robust tree.
But if the tree is unhealthy in the middle, pruning will just kill the tree faster.
This is where my mind was going with the metaphor I once took an Apple Tree Pruning course (yes, at college – a winter course elective), so I learned a lot. [I knew a lot from my grandmother but getting to the science of it was pretty great].