I don't think even the "artist" knows what she's talking about. She said "the structure is a 'layered history piece' that honors American Indians, immigrants and others who have lived over the centuries on the land now known as Baldwin Park."
"This land was Mexican once, was Indian always and is, and will be again." is from a poem written by Gloria Anzaldua. (You can read the entire poem here.)
Apparently, artist Judy Baca added the other part ("It was better before they came".) herself.
Baca said that Save Our State's complaint over one of the quotes was misguided.
"It was better before they came" was uttered by someone Baca described as "a white man from Arkansas [slick Willie, perhaps ??], a civic leader" who was complaining about the arrival of Mexican Americans after World War II.
"When it went on the arch, its ambiguity became profound," she said. "The 'they' could be any 'they.'"
I don't find it to be "profound", I think that Judy is just an airhead.