The U.S. ban on Tencent (WeChat) and Bytedance (TikTok) is a huge deal not because it changes things in the U.S., but because it potentially changes things *within* China.
It seems plausible that once the order comes into effect US companies operating within China won't be able to use WeChat, increasing the complexity of doing business by taking away access to the most common messaging platform. WeChat's pretty central to daily & business life.
A big question will be whether app sales within China also get affected. iOS and Android are the dominant platforms, which means Apple and Google could stop offering the WeChat app and its updates within China (this is more of a problem for iPhones given the App store model).
The absolute broadest interpretation of the order suggests US companies might be prohibited from advertising on Chinese apps altogether, which would potentially curb sales for a bunch of multinational companies.
Some people within China are also panicking that this effectively limits communications options to speak to family and friends in the US.
Messages circulating on WeChat (Weixin) say stuff like: "WeChat might soon be banned in the U.S. It will be like 40 years ago, when post and international phone cards were the only ways to talked to loved ones...' (H/T @danwwang) 
There's one more thing that is a pretty big deal here. Again via @danwang at Gavekal, the language used in the order is so vague that a broad reading could also suggest that in 45 days no US companies would be able to *supply technologies* to Tencent or ByteDance.
If true, the order would cut off a source of software and semiconductors for both the companies – including their subsidiaries (Tencent has quite a few!)
So effectively the US has just hit China in two in two very painful ways. The order has the potential to massively complicate and/or slow down China's technological ambitions. It also messes with things within China's own borders (which China hates).
