I think you're right that Thierry is not a genuine artist (hence him having a bunch of hired help) but I think the movie is about hype and as Shepherd Fairey sa

I think you're right that Thierry is not a genuine artist (hence him having a bunch of hired help) but I think the movie is about hype and as Shepherd Fairey says "perceived power creat[ing] real power." I think Banksy is playing a "prank" on the art world and showing how superficial they are. Thierry has lines about how he buys clothes for $50 and sells them for $5000, then Banksy makes the fake money that people spend as real money, there are other examples but basically the motif is that people easily fool themselves into thinking fashion/trends/art or anything that has no value, has value, and in doing so manages to create actual value.