I really disagree with that statement. Toy dogs in general are notorious for being hard to housebreak. At 12 weeks, in my opinion, and I’ve had many, many dogs, no dog is housebroken. It’s just not possible, their bladder is not mature enough.The good part of a toy sized dog is that they housebreak quickly. Often by age 12 weeks.
All my small dogs have been taught to go outside, even in out harsh winters at -40. Even my 4 pound chihuahua. But some took close to a year before being reliable.