E.g., from the article: "Dr Feigl-Ding explains that R0 is the “R reproductive number at time 0 before countermeasures”. He points out that this is not the R(effective) at current time under mitigation measures such as distancing and testing, tracing and quarantine, which are expected to slash chains of transmission."

Thanks for adding that. Interestingly, Eric Feigl-Ding is a bit like Nicholas Christakis in that he is not an expert in infectious diseases, however he has positioned himself as an expert on COVID19. This seems to have annoyed some of the real experts like Marc Lipsitch. However, Feigl-Ding probably does deserve some credit for sounding the alarm relatively early on COVID19.

The real experts were all saying ‘it’s too early to say for sure’ and he was saying, yes, but the worst case scenario is really bad and everyone should know that.

R0 is also culture dependent but before countermeasures are applied.

Higher R0 should reduce the effectiveness of measures like social distancing, shelter-in-place, etc too though - those work by reducing the number of people each infected person interacts with, but if those interactions are more likely to spread the disease than previously estimated the eventual R you end up with is going to be higher than expected too.

Exactly, like HIV started with a high R0 but its transmission mechanism is pretty easy to get a handle on--don't have unprotected sex or donate blood. Once this information is widely known, BAM, low R0. SARS-CoV-2 is quite a different beast, and reducing the R0 might be a hell of a lot more difficult.

There is no HIV risk in donating blood. The risk was in receiving blood transfusions, before blood banks started doing HIV testing.