From the actual release:

This decision follows Merck’s review of findings from Phase 1 clinical studies for the vaccines. In these studies, both V590 and V591 were generally well tolerated, but the immune responses were inferior to those seen following natural infection and those reported for other SARS-CoV-2/COVID-19 vaccines

Funny how your snippet leaves that 2nd part out. It's not that vaccines aren't effective, it's that their vaccine can't compete with the better vaccines that have already reached the market.

Anyways, the good vaccines have success rates in the 93%+ range 2 weeks after the 2nd dose, or ~95% a month after that 1-dose vaccine. For comparison, a typical flu vaccine is closer to 50%. And while that does mean you can take the vaccine and still end up dying from the real virus, if it prevents infection for 95% of the population, then the virus can't sustain itself, and will quickly die off once a majority of the population is innoculated.