If you have well controlled heart disease and get CV which triggers heart failure and subsequent premature death, I think CV should be recorded as cause of death.
The death, in your example, will have been caused by heart Failure and CV would be a contributory factor.
The cause of death is recorded in Part 1 of a Death Certificate and this may be split into a, b c etc with whatever is written against the lowest letter being what actually caused the ones above it.
Part 2 lists the contributory factors.
In this case the poor girl died of a heart attack. She tested negative for COVID-19 so how can it realistically be listed as a cause of death or a contributory factor?
You can't have a Coroner deciding on a particular cause of death without evidence to support that claim. Chloe having a cough isn't sufficient reason to decide the cause of death was CV. Coughs are present in many diseases.
Maybe (if she'd survived longer) a later test would have shown CV but no-one knows that.
It is tragic though and quite horrific that her family are being vilified on social media at this terribly sad time for them. Obviously they claimed she had COVID-19 in good faith.