this is nothing you should do, unexpected things may happen as it takes host stuff into the build
PROJECT=Generic ARCH=x86_64 scripts/create_addon vdr-addon is the correct cmd to build an addon since at least LE7
PROJECT=Generic ARCH=x86_64 is the default and always works for me. I only specify these values when building for non-x86_86 hardware such as arm.
After thinking more about it, I think you are right and I should explicitly specify these values always. Thanks for pointing this out! Maybe I was just lucky that it always worked, but I cannot be sure the correct compiler and linker was used.