They did, Bandai bought Sunrise back in 1994. This also caused the creator of Gundam Yoshiyuki Tomino to leave Sunrise at the time. He came back to Sunrise in 1998 mainly because the next anime he created after leaving Sunrise was the infamous Garzey's Wing.
The article seems to have misread Sotsu's wikipedia article.
https://en.wikipedia.org/wiki/Sotsu. The wikipedia article says that Sotsu has a relationship with Bandai Namco and in particular with the Sunrise subsidiary which the author thought meant that Sotusu owned Sunrise when it just means that Sotsu helps produce various Sunrise anime.
Sotsu is an advertising agency that has produced several Sunrise anime including Gundam, but Sunrise owned the IP, and when Bandai bought Sunrise back in 1994 that means that Bandai gained the Gundam IP then.
Here Wikipedia list Sunrise as being a subsidary of Bandai Namco