While we’d like to be able to give a vaccine to everyone, it can still be very useful to vaccinate even a small percentage of the population. Remember, COVID-19 kills less than one percent of the people who catch it, and it’s largely predictable who’s most at risk. So if we can get enough doses of vaccine to give it to 1% of the population, and target those doses at those at highest risk, that’d make a huge difference in preventing deaths.
And as the number of doses ramps up (though still less than the total population), it gets even better. Giving a vaccine to a fraction of the population decreases the chances of infection even among the unvaccinated portion of the population, especially if you target it to those most likely to catch and spread it in the first place, like health care workers.
No matter how hard we try, we probably won’t ever get literally every single person vaccinated. But we won’t have to.
All that said, if you read closely, there are a lot of weasel words in that article, like “preliminary” and “promising”. It’s good news, but this vaccine isn’t ready yet. Warp Speed, beginning mass-production before the testing is finished, means that if it (or any of the other many candidates) passes all of the tests, we’ll be able to hit the ground running, and start benefiting from it much more quickly than usual (at the cost of probably wasting money on mass-producing some that won’t pass the tests). But we do still need to finish those tests.