Hi Tim,
When used in connected mode, SonarLint uses the plugins from the SonarQube server. However, not all the plugins, only the ones created by SonarSource, which are verified to have acceptable performance in the IDE. This is why you won't see kotlin issues, because the kotlin plugin is not a product of SonarSource, and therefore not used in SonarLint. We are considering to allow such external plugins, but there is no concrete plan yet. On the other hand we plan to release a kotlin plugin soon.
Cheers,
Janos