That sounds like overreach; I don't think an employer is within his rights to try to restrict what you do in your free time with your own resources and without infringing on your employer's legitimate claims to IP (copyright, trademarks, patents, trade secrets, etc.)

This kind of overreach in contractual language and managerial posturing is somewhat common, driven by fears related to your exposure to your employer's IP, but the bark is worse than the legal bite here. I'm not a lawyer, but I still feel confident when saying that an employer wouldn't prevail in court, if they took legal action to enforce such restrictions on what you do outside of work.