That's not the case.
- people round numbers. In a casual setting 8 minutes and 10 minutes are the same.
- 10 minutes without detectable vital signs is survivable, albeit extremely serious.
- If you have sudden cardiac arrest and get CPR from a random bystander you have a 20% chance of surviving to leave the hospital. The sooner a defibrillator can restart the heart the better.
- I'm not sure why you say they should have still had vitals with a heart attack. Most heart attacks don't leave you with none, but it's not all by any means. Lack of blood flow to the heart can easily cause it to stop beating.
Ultimately though, the part that matters is that the part you found implausible was the medical details, not that someone would go back to work shortly after a severe emergency while still concerned about their own sudden death because they can't afford to recover.