It's outlined in their previous post on the Rope/SumTree data structure they use...

DylanSp 3 months ago | parent | context | favorite | on: Bit-twiddling optimizations in Zed's Rope

It's outlined in their previous post on the Rope/SumTree data structure they use, which this article links to: https://zed.dev/blog/zed-decoded-rope-sumtree.