The minimum difficulty required for a block to be valid is the thing that stays the same. Within that 2016-block window the actual difficulty of various blocks varies above that threshold.
It would be less ambiguous if you said, "Successful pools do not build on the first block they hear; they build on the block with the highest work they hear."
Well, we're splitting hairs here, but technically I might have gotten lucky and not worked very hard to find a block whose hash, in binary, ends with 250 zeroes in a row (outrageously high difficulty). "Difficulty" and "target" are actual technical terms with precise definitions in the block protocol.
But yeah we're talking about the same thing.