It's important for us to interpret data correctly and not speculate and draw our own conclusions.

The Slate article cites a few sources that say where cases are going up and where they're not, and have a snippit from a phone interview with a Climate and Health expert at Columbia University. This is not "data indicating that protests aren't contributing near as much as all the people indoors without masks", it's speculation.

Slate is not an unbiased source, and would have a strong bias towards justifying the protests.

I agree the protests are necessary and worth the Covid risk, but it's speculation and selective data interpretation to draw this conclusion at this point.