It seems that second version of _localizer["Thing {jep}", jep] is not using arguments at all.
https://github.com/damienbod/AspNetCoreLocalization/blob/master/src/Localization.SqlLocalizer/DbStringLocalizer/SqlStringLocalizer.cs#L36
Is there something I need to know about C# or is this just in todo phase. I believe that there should be few more lines in that method like this.

var localicedString = this[name];
return new LocalizedString(name, String.Format(localicedString.Value, arguments), localicedString.ResourceNotFound);