Just read through

A cytokine storm is an overproduction of immune cells and their activating compounds (cytokines), which, in a flu infection, is often associated with a surge of activated immune cells into the lungs.

Older people dying during the first wave seems very logical because most reports are that the dead elderly had other health issues that were taxing their bodies already. The elderly on the cruise ship who were healthy seemed to survive. Remember you can have multiple virus infections happening at the same time. You can also have a bacterial infection the same time you are fighting a virus. It wouldn't be wise to think this brand new virus only targets the elderly. This absolutely has the possibility to act like the Spanish flu did and you might want to look up the demographics affected by that and I think that's why bill gate is so worried.

Tl;dr Wuhan corona virus may allow other diseases to kill, it may be able to trigger deadly cytokine storm