I mean, sometimes it actually is pretty easy though, especially on a small patch job. Maybe that guy just had a simple fix and wasn't talking about a whole room. People only notice the bad work, so if you're good they think nothing happened.
You know, jake_patel has a point about people only noticing bad work. But the real trick is that good mudding makes the wall itself disappear, not just the seams. You're not hiding a repair, you're rebuilding the whole surface so it looks like nothing was ever there. That's the part that looks easy but takes years to get right.
I used to think good mudding was just about smoothing out the tape and not leaving a lump. But you're right, it's more like erasing the whole idea that a repair ever happened. The wall just looks like a wall again, and that's the part that sneaks up on you.