x86-64 vector compare instructions (mostly) produce masks, which are vector-sized values containing 1 for each bit of the input for which the comparison is true, and 0 where it is false. Since a vector analogue of x == x would be true everywhere, it would result in 0b1111....

It's true that the given C code doesn't have those semantics, but you can see what the GP was driving at if you squint a bit. Uh, and if you know how these instructions work already.