Failsafe to Altitude Hold from VIO(Position)

[hmlow]

Hi devs

Referring to the following statement from Seeker's and Sentinel's docs and with EFK2_AID_MASK == 280, EKF2_HGT_MODE == 3,

If VIO data is not being transmitted, the system will fall back to altitude mode and you will hear “position mode rejected, altitude flight mode”.

I understand that when VIO fails, the 2D x-y position hold obviously fails, but how about the altitude holding capability in Altitude Hold?

Does the Flight Core rely on any other sensors to maintain its altitude? Does it make use of its onboard BMP280?