That did it. I interpreted the line on this site saying "and as always, we've kept all our software backwards-compatible" to mean that current configurations (e.g. Stretch) would work. Apparently not. In any case, thank you for your help!

I am having the exact same issue. I've tried known good SDcards that boot just fine with my 3B+. I've tried multiple cables and multiple TV's and monitors. Using an official USB-C power supply, etc. Same exact situation with the lights, and it just says "no signal."