Adding a bit of friction reduces the number of people motivated enough to overcome it.
We're talking about people willing to kill. It seems obvious to me that these are the kind of people that are motivated enough to tap on their phone 10 more times.
People willing to kill is a small subset of people willing to forward rumors. Deliberate friction can indeed be useful.
Only a minority are willing to kill. The rest just lazily contribute to spreading false rumors. If you stop the lazy ones, you may stop the rumor from going viral and reaching the killers.
This change makes both fake news and true news hard to spread.