Most likely it’s a proxy because it’s some large farm. And the hashrate displayed is not accurate, it’s maybe one of the rigs that sends it to the pool while he has dozens of rigs in total.
There is no secret in here. Just his hashrate is hidden and since he found a bunch of blocks already it means that it isn’t luck.
Mining this way won’t make you find 1 ETH block a day with a GPU or 2.
Why do you say "likely"? It's a proxy definitely. Even pool detected this saying "proxy detected", This account is using a special proxy server that filters out low difficulty shares, only submitting shares that solve the block. Low difficulty shares goes to pool while high difficulty shares goes to solo and benefit proxy user solely effectively stealing from the pool.