It does not say that we can't produce terahertz. It says that there are no consumer products that make use of terahertz, because (among other reasons) producing it is more challenging than, say, producing microwaves. That statement is correct.

When people say "terahertz radiation", they do NOT mean visible light. They are referring to radiation between (roughly) 0.1-10 THz, which is much lower frequency than visible light.