if Garmin Solar Instinct is intrinsically safe

No its not IECEx/ATEX approved, it needs to be taken into account already during design of the circuit boards There are some brands that do offer it, like the Samsung Galaxy Smart-Ex Watch or the Pepperl+Fuchs’ Ecom watch. But sadly Garmin does not have a approved product line.There are some companies that do convert watches. https://atexxo.com/products/ex-proof-smart-phones/atex-watch-s5/ but I have never seen a real certified Garmin.

Even though that heart monitor is disabled?

I am afraid not. The display for example, is just regular strengthened glass on the Instinct. Where it needs to be a Sapphire or Gorilla Glass SR+ type of glass with very high resistant to impact. As mentioned the PCB board needs to be intrinsic safe. If it comes down to a incident the first thing the insurance will ask, is your watch ATEX approved?

Thanks!