I too am curious what is to blame for this warning and poor reference. It appears I have the exact same message in first aid. Also, I have had two start up disc failures 7 days apart, on top of some other strange things (like photos not responding). Current solutions in between startup disc failures were to complete first aid, reset SMC, reset NVRAM, and ultimately reformat volume and reinstall (clean) OS 10.15.x. I found that recovering from time machine ate up about 250gb of my 1tb hard drive. I did not think to check first aid on volume until after initial time machine recovery. I am concerned one or both parts of the fusion hard drive are failing.