I dont use TikTok, but its pretty clear to me there are some unique creations coming from it. Its absolutely not only about "lip syncing over existing songs". Here are some great examples of the remix phenomena:
TikTok lets you easily put your own video alongside an existing video, or reuse the audio from an existing video.
This basically gives you the tools to develop video memes. You'll often see a video or audio clip go viral and then see a bunch of different riffs on it that evolve over time, each a twist on the previous iteration, in exactly the same way you see image/text based memes evolve over time, get mixed together, adapted for a current news story, etc etc.
This sort of thing doesn't work quite as well on platforms like YouTube, the barrier to creation is too high.
Another component of the innovation is to mix lowering that barrier with a strong algorithm to surface the best content, since you now have so much of it.
I tried TikTok, it reminds me of when I used facebook about 8 years ago: full of relaxing/ funny videos, but leaving a feeling of emptiness having not done anything for the past 30 minutes. So I uninstalled it after 30 minutes of viewing. It's the opposite of Twitter, which induces stress and the desire to reply to someones tweet.
Though I wonder if there's any educational/ productive content on TikTok. This was someones argument about the difference between Twitter and TikTok: TikTok doesn't allow conversations/ debate.
Editing videos on mobile is really hard. Sitting on a desktop, transferring the videos, downloading the other videos and sounds you want to remix in, and stitching it all together in a video editing program is actually a really high barrier.
Tiktok is basically set up for remixing. You can splice your reaction, or add your dub, or lip sync, or more, and any of those would take less than 5 clicks to upload.