This assertion is contradicted by the hundreds of thousands of people (myself included) who have progressed beyond the beginner stage after starting out with the 20kg bar and without ever requiring the intervention of a human coach.

That said, I would have benefited from one. I had to completely deload and relearn my squat form because I was consistently leaning forward and de-emphasizing my posterior chain (now it's my best lift).

Speaking from experience, it's really pretty difficult to cause yourself an acute injury (i.e., worse than a nasty bruise) with 20kg if your form even resembles the squat, bench, or deadlift.

Granted, 20kg can be a big starting weight for overhead press, and if you're a petite woman you may initially need an alternative to the Olympic barbell even for the others.

Also, deadlifts are kind of tricky: a bare bar on the floor is a deficit deadlift. But a couple of blocks can solve this issue.