If what you said is the primary reason for banning TikTok (bad actors can do bad things), it's also a valid reason to ban literally every social network, or possibly even all user-generated content on the internet.
They can still kind of do it, but it requires a lot of work to fool other companies algo's into artificially promoting what you want. Much easier to just call up Bytedance and say "We need everyone in this area seeing this tiktok tomorrow".
To the second, you misunderstand the issue the US government has here. It is not that the social network is compromised and can be manipulated to any number of uses by an external authority. It is that it is compromised and can be manipulated to any number of uses by an external authority that they are enemies with.
Whether you consider them your enemy, whether they consider you theirs, whether you think that China really is or is not an enemy of the US government, and whether you consider the US government your enemy or not is all irrelevant to the point at hand, as interesting as they may be in other contexts; this is about the beliefs of the US government.
China has similar concerns and has already taken numerous similar steps, and it's equally not any sort of hypocrisy or anything because the principle they operate under is not about the existence of control, but who has the control.