That's not what it was about, read the article, catastrophe. At the very least just look at the graphs.
To your point, even if 15% of Sanders supporters don't support Biden, that doesn't seem outside of what would be considered normal anyway. The teeth gnashing comes from the fact that dems feel like everyone that doesn't favor trump should favor Biden and that isn't reality in any primary to general transition, no matter how awful trump may be to some.