I have heard that from coaches that beginners need more movements and honestly that makes no sense. Why would you start out with a bunch of volume and assistance exercises to maybe later do less work. If anything as you advance you must increase your overall volume to further get stronger.
You don't do more movements to somehow magically reduce the athletes workload later on. You do it to develop more coordination and awareness so it's easier and safer to teach them more complex movements later on.
Reducing things down to simple single joint exercises makes it a lot easier for an uncoordinated beginner to develop awareness of their body.
Imagine I'm teaching a front squat to a complete newbie who's never done anything athletic in his life and has zero muscle tone or coordination. When I tell him to contract his quads, will he know what to do? Probably not. But if I get him familiar with leg extensions beforehand, he'll know exactly how to do it. Not only that, during the squat he also has to contract his glutes while contracting his quads, while also actively maintaining a straight back, keeping his abs tight, breathing properly, making sure to stay upright, and keep his hamstrings from fighting against the quads. It may not seem that complex to people with some level of coordination, but for people with none they have to work through all of this consciously.
This example is using relatively simple joints. Imagine what happens when dealing with the shoulder girdle and the myriad of small muscles involved in those movements.
Until you've trained someone who's had zero athletic or physical experience outside of walking and sitting on their ass and no natural ability it's hard to really appreciate the level of coordination that can go into simple movements.
I remember learning this the hard way when I was teaching a group class of relatively experienced lifters the power snatch, with one guy who was completely new to everything. No matter what I did to try and cue the proper movements, he just couldn't make it work. Everyone else picked it up relatively quickly and looked passable. Dude had zero muscle tone and shit coordination. If he had just stuck with it he would have ended up badly hurting himself. Lots of young children are the same. I taught children's gymnastics before I ever trained an adult, and it gave me a good understanding of these principles because most kids are useless and uncoordinated.
In reality, approaching physical training this way makes it very similar to teaching anyone anything. If you want to teach someone piano, you don't start someone off with Beethoven's fifth. First they learn hot cross buns and develop some coordination and dexterity first through simple exercises, scales, and songs.