Siri (Apple Intellignece) doesn't work in clamshell mode

I just installed Sequoia 15.1.1 (Apple Intelligence) and Siri doesn't respond unless my Macbook pro M1 Pro is open. I have two Apple monitors attached, so I don't use the laptop monitor much, and I can see that the mic on the monitor can hear my voice, but Siri doesn't respond. I've read here on Apple Support that for past generations of MacOS, Siri needs the macbook open. Is that still the case? Why?