I had big legs before I started lifting. I'm 40 and have been riding a bike for about 35 years. The last 20 of them in very hilly places. My legs aren't huge and would be considered skinny in powerlifting circles, but are big in average joe circles. Again, that's from decades of riding a bike. Rowing will produce some muscle on a typical roadie. It won't be much and it will come on slowly, but it will be more than if you sat on the couch eating dorritos. But it's important to have realistic expectations and it's really not going to build much muscle.
Also, I think a lot of what people think of as muscular really comes down to having low enough body fat for the muscle to show. That might not apply in this crowd but it's a rule of them among the people you're gonna run into at the grocery store.