Trump looked like a fool when Paul reminded him that China is not part of the TPP
but most Trump supporters won't care
He never said China was part of the deal. He said something to the extent of "come in through the back door". And he's not wrong that if they try China could benefit a lot from the deal if they exploit it. We talked about it earlier in this thread.
The deal reduces the amount of material necessary to be considered originating in a country pretty much across the board. For example, under NAFTA, 62.5% of a vehicle has to be made in a member country. Under TPP, this drops to 40%. So more than half of a vehicle could be made in China and then sent to Vietnam (where China already owns a bunch of factories) for final assembly, and it would be considered made within TPP boundaries and subject to those tariffs.
Furthermore, there are a bunch of rules such the short supply list that could be loop holed.
Edit: His exact words were:
"It’s a deal that was designed for China to come in, as they always do, through the back door and totally take advantage of everyone."