No, it is not. Tsang is the Cantonese romanization of the surname 曾, whose Pinyin version is Zeng. When Zang is seen as a surname in mainland China, it refers to another Chinese character, which is 臧, whose Wade-Giles version is Tsang. So, you could see that this surname 臧 is written as Tsang in Taiwan. Here are several examples to illustrate what I mentioned.
Here is a picture of Margie Tsang, or 曾华倩 in Chinese, an actress from Hong Kong.
Here is a picture of Kenneth Tsang, or 曾江 in Chinese, an actor from Hong Kong.
Here is a picture of Zeng Li, or 曾黎 in Chinese, an actress from mainland China.
Here is a picture of Zeng Mengxue, or 曾梦雪 in Chinese, an actress from mainland China.
Here is a picture of Zang Xiaoyu, or 臧晓玉 in Chinese, an actress from mainland China.
Here is a picture of Zang Cong, or 臧聪 in Chinese, a singer from mainland China.
Here is a picture of Tsang Jui-Hsuan, or 臧芮轩 in Chinese, an actress from Taiwan.