Lots of us knew a long time ago. I think you may be conflating Woodward holding back on Trump’s knowledge with Woodward holding back on the information about the pandemic.
Plenty of medical professionals have been telling us for months about the severity of covid-19. That includes medical professionals who work for the US government.
I was reading about the risk as early as the beginning of February. I don’t think the danger was unknown or unknowable by the general public.
My dad had severe COPD before he passed away very recently. He and I were certainly aware by mid to late February how important it was for us to be masked and to take serious precautions to make sure he didn’t get the virus.
We learned to take those precautions through information available to anyone.
We didn’t believe Trump. I’m not certain that Woodward revealing his knowledge about Trump’s knowing denials would have made any difference back in February or March.
People who are disposed to believe Trump probably will, and people who assume he’s lying most of the time probably don’t need Woodward to tell them that.