Git and other resources are subject to US sanctions just like anyone else. They are the logical next step since the started making a fuss over mobile phone makers, networks providers, chip makers etc.
IANAL, but my understanding is that this is not true. Free software is not sanctioned. Commercial contracts that revolve around it are however. For example I think Huawei can still use AOSP, but cannot licence play services because they require a commercial agreement.
They can even force GNU open source to declare that this code is open source, except for non-democratic countries, such as China.
You, as an American citizen, are still under the legal jurisdiction of the long arm of federal laws.
They can probably even declare that all programs written in C or C++, are in effect, under American jurisdiction. Or even code that is encoded into x86 assembly, will be under American jurisdiction. And we know that all code written in China, uses the C programming language, as they have never developed their own technology stack.
In order to avoid this fate, then China must rip out everything from their country that has American technology, and develop their own technology stack, down to the ISA (Instruction Set Architecture). They cannot even use ARM or RISC-V, they must develop their own ISA, and call it CNA-1.
Then, they would have to develop their own programming language on top of this. Then their own operating system, and application software, to replace all American technology.