However onthe tensorflow installation guide (使用 pip 安装 TensorFlow), I only need to call the package from the package manager (apt install libcudnn7) without signing up for the Nvidia developer.
Why is there such a difference? Am I missing something?
In general, for the most up to date CUDA/CUDNN/etc. versions as well as for installing archived versions of CUDA/CUDNN/etc, the NVIDIA developer page is the place to go. It usually offers multiple installation options such as .deb, .run, tar.gz, network repo, etc. for different workflows.
The step to add the network repos are actually listed on that TF page as well here with the wget commands, which they are pulling from the NVIDIA Developer page: TensorFlow'u pip ile kurun
There may have been some exceptions made for login to allow for easier TF installation, but in general you can just go through the Developer program to get access to all of the options.