Why is Kong using a Japanese surname?
The keyword is in the (Japanese) naturalization.
Kong Wenge took a Japanese nationality by mean of naturalization. After naturalization, he needed to have a Japanese legal name.
From this article,

Do you have to take a Japanese name if you naturalize?
[...]
The simple answer to this question is YES, you do need to take a Japanese name.
The reality, however, is that a &quot;Japanese name&quot; is not necessarily what you are thinking. You have to follow the same rules that a Japanese parent has to follow when naming a baby... [...].
[...]. As you can see, some of us (like me) choose to write our existing names in Japanese characters. Others choose a more Japanese-sounding name that is similar in either sound or meaning to their original name, and others choose something that is altogether different from the name they were born with.
You do need to write your name in Japanese characters. This can include ひらがな (hiragana), カタカナ (katakana), 常用漢字 (jōyō kanji) (kanji for everyday use), and/or 人名用漢字 (jimmeiyō kanji) (kanji designated for use in names), [...]

(Emphasis mine)
While 孔 (kō) is actually an acceptable kanji (it's inside kanji for everyday use), given the privilege to change his name, he chose 辻堂 (tsujido), probably to show his support to his high school.