The engine option tells knitr which language to use to actually run the code snippet.
In the release notes, they mean syntax highlighting in the source editor. Open the existing Python source file in the editor and you will see that it is correctly selected.
The allocation of language-dependent syntax is usually selected based on the main language of the file being edited (often simply based on the file name extension). In your example, you are editing the .RMd file, so the editor uses the syntax rules for r-markdown.
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