@jesse Oh! I believe both Karim and I took https://developers.mattermost.com/extend/plugins/server/hello-world/ to be the source of truth despite our version differences.
However, the logs does show that it was successfully able to activate the plugin though:
{"level":"info","ts":1533055427.971367,"caller":"app/plugin.go:208","msg":"Activated plugin","plugin_id":"com.mattermost.server-hello-world"}
Probably that’s what threw us both off track thinking “We’ve done everything right and there’s something wrong with the doc”.
Will definitely keep the 5.2 upgrade in the pipeline. However, for the time being, at least I have to work with 5.1 since we’ve deployed and it is fuelling business logic in full swing. 
Thanks for the plugin example for 5.1. Going to give that a shot right away and will report back on the thread with an update.
Thanks again @jesse, @amy.blais and @karim!