I missed that, but it's pretty unusual for fourth round picks to get cut or not make the 53. There's nothing in a rookie contract that guarantees a player will make the roster but generally teams don't give up on draft picks that quickly.
Also, he's taken part in two preseason games (one of which he played a majority of snaps in). And the camp reports are about what we expected - highlight catches mixed with drops. He's a high ceiling project. He's buried on the depth chart but the Cardinals are going to play in a lot of five wide sets so it figures that they'll keep six or seven on the active roster.
The OP said maybe he isn't NFL caliber. Change that to NFL ready right now and I might agree with you. And FWIW, most rookie WRs aren't major contributors right away. It's one of the most difficult positions to transition from the college game to the pro game.