You're right: in 6.2.x, injectVars were part of uiExport.app. The Kibana Platform team removed that touchpoint in prep for the 8.x Kibana framework, which won't have uiExports.
To check on that, I'll again direct you to the Timelion code, this time going back to our 6.2 branch: https://github.com/elastic/kibana/blob/6.2/src/core_plugins/timelion/index.js#L11