I can see tegra_defconfig from r32.6.1 kernel sources.
for example, $L4T_Sources/r32.6.1/Linux_for_Tegra/source/public/kernel/kernel-4.9/arch/arm64/configs/tegra_defconfig
I did correctly find tegra_defconfig in several releases. Which exact release did you download? If I have that, then I can try to mirror downloading that release and look for the tegra_defconfig.
I do have to say that your procedure looks correct, but since the sources I’m looking at have the file, then I wonder if there is something about your kernel source itself which is incomplete.