if it's before though, then how would isolation after symptoms prevent the spread? I think the original post stand if perhaps overstated.

Not symptoms, but positive test results...

In the US, people are going to work (or other events) WITH positive test results. Some because they won't get paid otherwise, others because they want to make a political statement, and some because they're ... well, I dunno why...

So, yes, even having people stay home after a positive test would be helpful, because currently, they aren't even doing that.