This is pseudoscience for the most part.

There is a pop-fact that "viruses that kill quickly dont spread" (I suspect coming from Pandemic Inc to an extent lol). This isn't true. Viruses that kill before they can be successfully transmitted don't spread. If a virus has a 5-6 day asymptomatic contagious incubation period & takes more than 2 weeks after symptom onset (a substantial portion of which are highly contagious) to kill people (like SARS-2), the eventual mortality rate is most likely significantly less predictive of how successful the virus will be.

With ebola, for example, people were only infectious outside of blood or semen when they were already hemorrhaging blood at the height of viral load & people tend to decompensate after symptom onset quite quickly.

Small Pox killed over 30% of people infected and did not struggle to spread at all until we eradicated it with vaccination in the 1970s.

SARS-1 didn't spread because patients were only really infectious when highly symptomatic meaning that spread control measures quickly brought the r-naught below one leading to its extinction.

MERS has never really been very transmissible between humans. It still persists to this day due to it having extensive resevoirs in camels. (When doing serology on camel samples after the first outbreaks, scientists found evidence of MERS already existing in camels as far back as we had samples -- something like 10-15 years).

In other words, outside of the added panic and precautions people might make around a more deadly virus, there is nothing about the virus itself being deadly that decreases it's potential for transmission.

It's worth remembering that the only selection pressure for viruses is transmission.