Looks like you visited Rosario Head!
What you've got here is a Surfbird - plumper, a bit larger, and with a more even coloration on the back. The flank streaks form neat thin chevrons on a Surfbird, as opposed to the more dense, spot-like look on the flanks of a Rock; finally, a look at the bill would reveal a clear difference: a Rock Sandpiper has a thin, average yet decurved bill, while a Surfbird appears to have a thicker, stubbier bill. (A direct objective comparison between the two bills would reveal a similar length -- however, the larger size of a Surfbird, especially in the proportion of the head, makes the bill seem shorter in comparison.) Another thing to note about the bills is that the Rock has orange at the base of both the upper and lower mandibles of its bill, whereas a Surfbird only has color on the lower mandible. Let me show one of my own photos (from what I presume is the very same location as your bird, Rosario Head) to highlight key differences:
Left bird is Rock:
Rock Sandpiper 1 by raven_lunatic11, on Flickr
Bottom left is Rock:
Rock Sandpiper 2 by raven_lunatic11, on Flickr
Perhaps the birders misidentified the Surfbird as the continuing rarity -- or perhaps they were training their scopes on another bird? Do your photos reveal any other shorebirds?