PX4-Autopilot: RC PPM input not working on HolyBro Pixhawk 6X with IO off

If I set SYS_USE_IO to 0 on a Pixhawk 6X, I no longer receive RC.

nsh> rc_input status
INFO  [rc_input] Max update rate: 250 Hz
INFO  [rc_input] UART device: /dev/ttyS5
INFO  [rc_input] UART RX bytes: 8
INFO  [rc_input] RC state: searching for signal: ST24
rc_input: cycle time: 6594 events, 41091us elapsed, 6.23us avg, min 4us max 967us 16.218us rms
rc_input: publish interval: 0 events, 0.00us avg, min 0us max 0us 0.000us rms
nsh> rc_input status
INFO  [rc_input] Max update rate: 250 Hz
INFO  [rc_input] UART device: /dev/ttyS5
INFO  [rc_input] UART RX bytes: 8
INFO  [rc_input] RC state: searching for signal: PPM
rc_input: cycle time: 6720 events, 41742us elapsed, 6.21us avg, min 4us max 967us 16.066us rms
rc_input: publish interval: 0 events, 0.00us avg, min 0us max 0us 0.000us rms
nsh> rc_input status
INFO  [rc_input] Max update rate: 250 Hz
INFO  [rc_input] UART device: /dev/ttyS5
INFO  [rc_input] UART RX bytes: 8
INFO  [rc_input] RC state: searching for signal: GHST
rc_input: cycle time: 7076 events, 44060us elapsed, 6.23us avg, min 4us max 967us 15.948us rms
rc_input: publish interval: 0 events, 0.00us avg, min 0us max 0us 0.000us rms

With SYS_USE_IO set to 1, the IO board correctly has RC:

px4io status
...
8 raw R/C inputs 1500 1502 1505 1506 992 991 992 1503
...

This is with an FrSky D4R-II receiver jumpered to output PPM.

I looked into this related to this question: https://discuss.px4.io/t/forwarding-rc-channels-through-i2c/31334/6