Rad-ndo bikes

I also found this company that says it works: https://www.rodbikes.com/articles/9-10sp.html

On some of our touring bikes and tandems we use 10 speed Campagnolo shifters with a 9 speed SRAM or Shimano cassette in the back. We're not the only high-end manufacturer that does this, but we are probably the biggest.

So I choose to be optimistic. It's free to try since I have a 9sp stuff laying around already, worst case I buy new 8sp cassette and chain.
I love downtubes, but I'm curious about whether this would work, and I only ever rode brifters for like 5 minutes, and those were some ratty Soras. I'll still carry a pretty friction dt in a bag for when the campy shifter fails or whatever. I'm paranoid enough.