There is no doubt in my mind that what they are saying is true.
Shooting nature is a whole different ballgame than shooting weddings, kids and senior photos. With them in most cases you are in control of the light. If in a studio you have your lights and if shooting them outdoors some open shade and or a reflector takes care of most of what you need to do.
Shooting nature / landscape the only control over that one big light in the sky is the time of day. Even there nature controls how much of that light you have access to and what color that light is.
You can't shoot a sunset over the Pacific Ocean if you are only on the beach in the morning or mid day. You have to be willing to work late but work quickly because the light changes so fast.
You can't get a good shot of a rapids or waterfall and not end up with blown out highlights or black shadows if you shoot them in the middle of a sunny day. You have to shoot when it's cloudy or in the morning - evening when the sun is off the water.
You have to be out there at the "good" times of the day to get shots that are different than the vacation snapshots that anyone can get. It's kind of like hunting. You get up early, take a nap mid-day and get back out there in the late afternoon. You have to make the light work for you and take it for all it's worth.
The more often you are out there and shooting at the right time of day the better your chances of getting those "WOW" shots. Remember WOW = $$$$$.