From their README:
Minestom is an open-source library that enables developers to create their own Minecraft server software, without any code from Mojang.
The main difference between Mojang's vanilla server and a minestom-based server, is that ours does not contain any features by default! However, we have a complete API which is designed to allow you to make anything possible, with ease.
This is a developer API not meant to be used by end-users.
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I believe the target audience is people developing non-vanilla/survival multiplayer servers, like minigames, PvP duels, etc.