Garmin watchs will be able to detect Covid?

No watch is going to detect Covid-19, or any other illness/disease for that matter. They can detect a change in HR, respiration rate, stress, etc. which could mean (major emphasis on the word could) the user is becoming ill. It could also mean you are over training. 

this is not what they say in this article:

blog.fitbit.com/.../

They're basically using a metric of HR & Respiratory rate and drawing a conclusion.

Long been known that a change in rest HR can indicate added stress to the body, whether that be seasonal cold, flu, heat stroke, etc.

The article's you're posting are misleading in their wording. They're doing nothing more than filling in the blank after the fact.  As I said earlier, these watches can detect a change in your biometrics and sometimes if you're becoming ill they can see these changes before you feel ill.  Just because you now go to the doctor and get properly diagnosed with covid, influenza, Lyme disease, hypertension, scarlet fever, (fill in your own blank), doesn't mean your watch detect these illnesses. It detected a change in your biometrics and nothing more. As I also said earlier, these changes could be from overtraining also.  The day a fitness watch has a pop up that says, covid, flu, etc. and be correct is when it can be said to have the ability to detect illnesses.