It doesn't show, but that whole thing getting dismissed left him with a slight taste of disappointment. He thought it'd sound cool (and little does he know, it actually was,) but the whole thing getting dismissed over the tree stung his ego a little.

"Because it's not real?" That seemed like the obvious answer to him. Which was a good reminder that he should probably put it back before Yona realizes something happened. Which is when he goes to pick the chucked tree back up and carry it over his shoulder like it was- well, not nothing, but just a small burden. "It ain't even heavy, so there's no way that it's real. Sure, it's convincing, but it's not like I could rip up an actual tree."

It sounds like the most obvious truth there is, if only because that's what Shizuo thinks is the case. What else could it be?