There research is not credible because their only "evidence" is that when using certain keywords, Instagram and YT returned more "anti-China" content than TikTok.

So instead of arguing the U.S. social media has an anti-China bias, they argued that it's the evidence of TikTok being more pro-China.

Using American social media as the control group for neutrality on China is absolutely insane.

The most likely cause is that TikTok is just a lot less political and more international than YT and Instagram.

For rest of the world, people do not automatically associate words like "Xinjiang" to "Chinese government oppression", the fact that they expect that to be the top result can be argued that American media is the one manipulating information.