I think most Americans are already internally aware to different extent their chances in any kind of war with China is bad, it's the whole reason China lives in their head rent free and why China collapse videos are so popular.
I dunno why but this tweet is blowing up. The comments and quoted tweets are very interesting, especially the praises from Indians? I feel like this message has been said by China's MFA before, maybe the Americans online are anxious now, they are no longer confident in their comprehensive strength.
So when China directly said "bring it on b*" to their face, it basically confirms their fears and hits that inner voice of theirs especially hard.