What is the difference between Qt bundled harfbuzz and upstream harfbuzz?

[Paper]

I want to build Qt 6.3.1 on macOS with system harfbuzz, but I got this warning:

On OS X, AAT is supported only with -qt-harfbuzz.

So I inspect the directory src/3rdparty/harfbuzz-ng and know Qt uses harfbuzz 4.2.1 and imported by script import_from_tarball.sh. I download harfbuzz 4.2.1 from upstream and use this script generated the source, but I found there is no difference between the src directory generated by script and Qt's harfbuzz by diff, the other files are some dummy headers and sources, and import script doesn't change the source files.
So this warning makes me confused.

[ChrisW67]

You have demonstrated that the code bundled with Qt is unmodified upstream code. If you do not use the upstream code then you default to the Harfbuzz library supplied in OS X. The Harfbuzz library supplied by OS X is either a different version that does not support AAT, a modified version that does AAT in some incompatible fashion, or is in some other way incompatible.