You would have to publish the infected package first to infect others who haven't pinned their dependencies. With a simple pip install -U, and if the dependency is not pinned, then they will get the vulnerable version.
It's "if I pin the dep, I know that someone won't compromise the package repo and the next time I install 2.6.3 I can be sure that the same package is getting downloaded and installed."
This specific risk isn't just not having things version pinned. It's not having a hash of the package to check against to make sure you're getting the same package every time.