Custom Ribbon Button on Wiki Page's Editing Tools Ribbon in SharePoint Online

Using the Core.RibbonCommands sample add-in from https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/add-a-custom-ribbon-to-your-sharepoint-site , I was able to add custom tab into pages in SharePoint except while viewing/editing SitePages or publishing-pages.

For instance, while trying to add a button into Wiki Page's Editing Ribbon, the custom ribbon buttons just don't appear if I use the exact same xml from https://social.msdn.microsoft.com/Forums/sharepoint/en-US/d37f13ce-210f-4b6f-8a70-74544d362c72/custom-ribbon-button-on-wiki-pages-editing-tools-ribbon

Appreciate your help on how the ribbons can be customised for a Wiki Page