MQTT is just a messaging protocol, it’s mostly used for iot purposes like you mention, but can in theory be used for any kind of project.
Never used ejabberd, but one of their selling points are multiprotocol support, so it’s not so weird to support a multipurpose standardized messaging protocol.
Too late, learned the hard way; lost my first 6 hours after a crash. Probably also the first game that I can think of in this genre that comes with autosave disabled as default. The game is nice, but sometimes feels a bit like a full price early access title.