Article should probably explicitly call out the difference between directly attached nvme storage (good ol i3) and “nitro nvme” (m6id and friends). The later is provided via an embedded card which emulates/provides a virtual nvme device directly to the host/instance. Without digging in to the specifics Im oretty sure thats accounting for the $/perf numbers being relatively flat. And “i” series being local storage cost/perf optimized compared to other families.

Edit: see https://d1.awsstatic.com/events/reinvent/2021/Powering_nextg... and similar talks. And notice the language around benefits of more consistent performance due to the better mediation of resources.