I can't help but to have developed a cynical view on this. I think they realized that herd immunity/70%+ wasn't going to statistically happen nearly quickly enough, if at all, the way things are going, so they have changed the messaging in a last ditch effort to try to increase willingness for people to get the vaccine. I think people forget that the goal of the CDC is to handle the messaging to get people to behave how they want us to behave. The messaging is well-planned and coordinated - it was the White House announcement that was the most "slick" and clearly "marketing material" more than anything else.
That's really the only reason I can come up with for what changed between the beginning of the week to mid-week, given that we haven't even hit 50% vaccinations yet.