Wasn't the NVIDIA "Denver" Tegra K1 originally intended to be an x86-compatible chip along these lines, then when they couldn't get the licensing right, it was turned into an ARM-compatible?
Likely they developed a very fast, very efficient core dye. And just swapped decoders at the last minute. Its not much of a stretch that they'd develop the ARM set in parallel b/c of the high risk associated to doing anything with x86.