The "security issue" expressed is that someone creates 4 billion lock files. The entire reason an application would have a path to create these lock files is because it's dealing with a shared resource. It's pretty likely that lock files wouldn't be the only route for an application to kill a system. Which is a reason why this "security issue" isn't something anyone has taken seriously.
The reason is much more transparent if you read between the lines. Systemd wants to own the "/run" folder and they don't like the idea of user space applications being able to play in their pool. Notice they don't have the same security concerns for /var/tmp, for example.
The "security issue" expressed is that someone creates 4 billion lock files. The entire reason an application would have a path to create these lock files is because it's dealing with a shared resource. It's pretty likely that lock files wouldn't be the only route for an application to kill a system. Which is a reason why this "security issue" isn't something anyone has taken seriously.
The reason is much more transparent if you read between the lines. Systemd wants to own the "/run" folder and they don't like the idea of user space applications being able to play in their pool. Notice they don't have the same security concerns for /var/tmp, for example.