Confirmed. I have stood at the side of the road to flag down an ambulance for a critically ill person. That has at least as much to do with unmarked entrances on semi-rural roads as the lack of a postcode, though.
Your phone probably knows what your lat/long are, but can you quickly look it up in a high-pressure situation? Probably not. And emergency call systems such as 911 don't get that information either. Maybe they will in ten or twenty years, but they don't now.
If easy reliable access to that information is really the main barrier, it does suggest an alternative.
This works well for things like traffic accidents, where the caller may not know exactly where they are, but if you know the rough location, they're pretty easy to find (just look for the smashed up cars...).
It is a _lot_ less useful for a caller inside a house. We occasionally get dispatched to a geolocated address when a baby-sitter or some other visitor who doesn't know the house number calls from their cell phone. Unfortunately, even with the best accuracy available, it's still anybody's guess which house they are actually in (even in a suburban area, there can be several residences within the geolocation radius).
If they can get outside to flag us down, great. If not, it's a lot of knocking on doors until you find the right place...