The KendoGridSelectBy directive alongside binding the selectedKeys property act as a separate selection mechanism and regardless of the Grid "selectable" setting (that only has meaning when used as a standalone means for enabling/disabling the Grid default built-in selection functionality). When the mechanics above are used to customize the selection functionality, the selectable property is ignored:
Here is the updated StackBlitz where the kendoGridSelectBy directive and selectedKeys are omitted from the Grid configuration and [selectable]="false" has the expected effect:
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