Help me understand then if they’re following the letter of the law what changed with the law between the shutoff and now?
Well, "the law" is a shorthand for "how the police behave" and there is a certain amount of realpolitik here. The basic argument here would be that the US Congress made a scary growling sound and TikTok folded immediately because the Congress is terrifying. But then Trump made more of a friendly sound and so they think they can operate a bit longer with some level of safety.
There is no question that TikTok is a politically sensitive app and the US/China are very nearly in the funnel to a major war so a lot of the usual niceties are questionable. Previously the US has attempted something that looked a lot like a black-bag kidnapping of a Chinese industrialist [0]. I'd imagine that the TikTok people are acutely sensitive towards how the law is actually going to be interpreted and enforced in practice.
This is basically the same tactic to the SOPA/PIPA protests [1]. I don't know why people are bending over backwards to pretend it was something other than a political stunt. Also, Trump's rhetoric has remained unchanged since well before this - a 90 day extension. They wanted to flex their muscle to show the US political establishment how many US users there were and how much sway they had to give them more leverage in their negotiations. That's about it.
[1] https://en.wikipedia.org/wiki/Protests_against_SOPA_and_PIPA...
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