I think it's safe to assume that if he was wearing the merch of a youtube channel, then he was subscribed to the channel, or at least very familiar with the channel's content, yes.
If he were trying to blend in, it'd be more logical to just wear a Trump shirt rather than a somewhat obscure youtube channel. It's been a few days, and the shooter's conservative political leanings have been well established. Safe to say this was an accurate assumption.