Baloney. You can walk around all day long with your fingers on your neck or wrist. Have fun with that.
What do you think happens when your HRM watch runs out of battery? Yep, you resort to reliable two finger method.
You're probably also not even aware that HRM watches don't continuously monitor HR and only does it in intervals to preserve the already short battery life. For comparison, two finger method works as often as you want with no worry of draining battery.