Hi @sns ,
Per my test, after importing the .html file will not show in SharePoint Designer. But you could find this file in the bottom of Design Manager.
1.Download files in GitHub.
2.Import .html file in SharePoint Designer.
Master Pages > Import Files > Add file... > select the .html file > ok
3.Convert the HTML page to Master Page using Design Manager.
Settings > Site information > View all site settings > Look and Feel > Design Manager > Edit Master Pages > Convert an HTML file to a SharePoint master page > scroll down the new .html file is in the bottom
My test result:
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