It is usually a good idea to read a book before getting involved in controversy about it. The book is “The Upright Ape”, by Dr. Aaron Filler.
My interest in pre Homo sapiens fossils goes back to university days fully 70 years ago, when I could name and describe every one then known, and throughout that time I often read that there is no fossil leading to the existing great apes, so it did not surprise me to learn from Dr. Filler that no fossil evidence for a quadrupedal higher primate has ever been found.
What did surprise me was the evidence that chimpanzees and gorillas have backbones designed for bipedalism, and about the chimpanzee I was able to check immediately in my copy of Dr. Daris Swindler’s ”Atlas of Primate Gross Anatomy.” Sure enough, the chimpanzee lumbar vertebrae, just like the human, is a close match for a corresponding fossil found in Africa and dated at 21 million years—all of them differing fundamentally from the vertebrae of a quadrupedal baboon.
Most paleontologists are not also spine surgeons, so it is no surprise that they have missed what was obvious to Dr. Filler and continue to believe and teach that human ancestors walked on four legs until they became upright as tree dwellers, and then invented true bipedalism after returning to the ground. It is a deep-seated belief, but that is all it is. There is no evidence for it whatever.