What I mentioned were CPU threads, not OS threads. Scheduling a different OS thread on a CPU thread requires a context switch for the given logical core aka CPU thread. a context switch is an expensive operation, the old OS thread state has to be saved and the new OS thread state has to be restored.

The more CPU threads are available, the less often the OS has to switch a different OS thread on a CPU thread and thus the fewer context switches there are.

On a system with many OS threads that have a low CPU demand per OS thread, like a web server or an OLTP database with many short queries, this is a large part of the reason why an HT CPU outperforms a non-HT CPU with the same number of physical cores.