Marshall was a panelist on the ArrayCast podcast (https://www.arraycast.com/) which discusses all things relating to array languages. He's sadly missed :(
ArrayCast is a great podcast and I've enjoyed it quite a lot since it started. Listening to people talk about APL/K/J/BQN/etc is a great way to learn about the array languages. Maybe it's just me but hearing people use the glyphs in conversation went a long way to building intuition with how to think in arrays, so to speak.