Something like TPP improves the bargaining power of its multitude of smaller members whereas bilateral trade agreements favor the largest participant as it can divide and conquer.
I believe Trump, in his mind, would rather throw his weight around against the German auto industry for example and "cut a deal" with Merkel rather than with the entire EU.
http://www.businessinsider.com/trump-trade-merkel-germany-eu...
The US has had an enviable trading position ever since Bretton Woods but the world has caught up. The US being unfairly squeezed or experiencing fair competition is a matter of perspective I suppose.
The Bannons of the world are not wrong to notice the US lead is shrinking and something has got to give - but perhaps their solutions are a bit naive.
There seems to be no substance to this argument. It fits the pattern of Trump attacking something that's on balance good, but which is complex enough to easily find things to attack if you are entirely without a moral compass and just want to undermine your opponent. Then proposing to do the same thing "better" in some vague and unspecific way. Then failing to do so.