A "bothsidesism" is presenting two viewpoints as equal. This section of the article repeatedly states that the intention is to show how partisanship impacts potential belief in conspiracy theories. No where does it claim to compare the theories and beliefs 1-to-1 . And no, Bush/the government didn’t know about 9/11 in advance from that briefing, they had intel that bin laden was planning “attacks by hijacking planes.” The polls referenced here were framed accordingly. Like I already replied to you on Twitter, this is a bad faith interpretation from someone with clear motivated reasoning.

https://www.brendan-nyhan.com/blog/2009/08/911-and-birther-misperceptions-compared.html