If I understand correctly the code disabled plugins are considered installable:

https://github.com/jenkinsci/optional-plugin-helper-module/blob/2994c72c377d05eab48793e94dbbdcdf238ae79f/src/main/java/org/jenkinsci/modules/optpluginhelper/PluginHelper.java#L297-L308

Specifically, the line of code below should also check that the plugin is not disabled.

https://github.com/jenkinsci/optional-plugin-helper-module/blob/2994c72c377d05eab48793e94dbbdcdf238ae79f/src/main/java/org/jenkinsci/modules/optpluginhelper/PluginHelper.java#L303