Well, I've made further research on this subject.
The reason for this actually working is that, despite the codec files are installed inside the wrong folder (ie, %WINDIR%/SysWoW64), the installation info file specifies the correct registry key for installing a 64-bit executable on a 64-bit operating system:
[HUF.Reg]
HKLM,SOFTWARE\Microsoft\Windows NT\CurrentVersion\drivers32,vidc.HFYU,,huffyuv.dll
HKLM,SOFTWARE\Microsoft\Windows NT\CurrentVersion\drivers.desc, huffyuv.dll,, "Huffyuv lossless codec"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,Description,,"Huffyuv lossless codec [HFYU]"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,Driver,,"huffyuv.dll"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,FriendlyName,,"Huffyuv lossless codec [HFYU]"
So, regardless the codec files are actually installed, the operating system will find them when required by a 64-bit application. That is, the procedure described will work, but it's not elegant and will raise problems if you're going to install both the 32-bit codec and the 64-bit code in the same machine.
As a matter of fact, the 32-bit version of the codec, as provided, won't work at all in a 64-bit operating system, for the registry section of its .inf file goes like this: (see note below)
[HUF.Reg]
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,Description,,"Huffyuv lossless codec [HFYU]"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,Driver,,"huffyuv.dll"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,FriendlyName,,"Huffyuv lossless codec [HFYU]"
HKLM,"Software\Microsoft\Windows NT\CurrentVersion\drivers.desc",huffyuv.dll,,"Huffyuv lossless codec [HFYU]"
HKLM,"Software\Microsoft\Windows NT\CurrentVersion\Drivers32",VIDC.HFYU,,"huffyuv.dll"
That's ok for a 32-bit operating system. For a 64-bit operating system,they should be like this:
[HUF.Reg]
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,Description,,"Huffyuv lossless codec [HFYU]"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,Driver,,"huffyuv.dll"
HKLM,SYSTEM\CurrentControlSet\Control\MediaResources\icm\VIDC.HFYU,FriendlyName,,"Huffyuv lossless codec [HFYU]"
HKLM,"Software\Wow6432Node\Microsoft\Windows NT\CurrentVersion\drivers.desc",huffyuv.dll,,"Huffyuv lossless codec [HFYU]"
HKLM,"Software\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Drivers32",VIDC.HFYU,,"huffyuv.dll"
So the right, canonical procedure for installing the 64-bit codec would be exactly as described in the video, but copying the code files inside the %WINDIR%/System32 folder, usually the C:\Windows\System32 folder in practice. Of course, I have checked it in my laptop running Sony Movie Studio 64-bit on a Windows 10 64-bit OS.
PS: on a second thought, I believe is actually possible to install the 32-bit version of the codec on a 64-bit operating system as provided, as long as you invoke the right version of the setupapi.dll tool, ie, the one inside the %WINDIR%/SysWoW64 folder.
To do so, this command must be executed from an elevated console window:
rundll32 C:\Windows\SysWOW64\setupapi.dll,InstallHinfSection DefaultInstall 0 the_path_to_the_huffyuv.inf_file