Don't modern versions of Windows do the same? For example, I clearly remember that the Windows 10 installer first launches a Windows 7-like environment.
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Windows Vista and newer launch a more substantial version of the OS with the graphics system and Win32 services running, but they never intermix versions. Windows 10's DVD loads Windows 10 to run the installer. That they haven't updated the pre-baked Aero graphics since Vista is a laziness problem, not indicative of being "actually Vista/7" :)
And yet, you can use the Windows 11 installer to install Windows 7 and have it be significantly faster because of that.