I think I found the problem.
This project does not use Swift, but it does use il2cpp.
And the version of the Unity editor you use supplies macOS executables for the il2cpp compilation step, and copies them in a subfolder of the Xcode project directory. These macOS executables are supposed to be called by Xcode in a "custom build step" phase to compile the il2cpp static library that your project will use.
The problem is that these macOS executables are provided in two variants (Intel and ARM64, to support all Mac's hardware architectures, Intel and Apple M1/M2) *and* rely on dynamic libraries that are written in Swift.
So, the builder parses the Xcode project directory, discovers these files, identifies them as ARM64 Mach-O libraries, and thinks: this iOS project supplies precompiled ARM64 binaries, I should take note of those and include them in the final application. And so it does.
Consequently your final .ipa gets extra files embedded in it, which are useless from the iOS app's point of view because they aren't iOS libraries but *macOS* libraries, that doesn't prevent the app to run (these extra files are simply ignored), but when Apple receives them on App Store Connect for analysis, they do the same mistake as me: they believe these libraries are useful for the app (for what else could they be here?), identify them as Swift libraries, deduce that the app uses Swift code, check whether the SwiftSupport directory is present... and fail.
The "quick and dirty" solution, in your case, is to use a pre-packaging script that contains these lines (WARNING: THIS HACK IS ONLY VALID IF YOUR APP DOES NOT EMBED ANY DYNAMIC LIBRARY):
% prepkg.bat
rem // delete the extra il2cpp host compiler libraries that were mistakenly bundled in the app by the build script
del /f /q "%TARGET_BUILD_DIR%\%NAME%.app\Frameworks\lib*.dylib"
%
Now, if you rebuild your app, you should notice that the .ipa size is smaller as it doesn't contain those extra files, and there are chances that, if you bump the build number as well, Apple will accept your build on App Store Connect. Let me know please.
Meanwhile I'll be looking for a more "proper" way of fixing this.