I think a more likely explanation is that they supported everything that fits in 128 bit registers (the same as ARM NEON). Trying to emulate code using 256 bit

I think a more likely explanation is that they supported everything that fits in 128 bit registers (the same as ARM NEON). Trying to emulate code using 256 bit registers on a machine with 128 bit registers is probably difficult and pathologically slow in some cases.