I am not sure about the validity of this theory, as has been explained it has been disputed and debunked on several occasions.
However, a few important or worthy things that pop up in it that have been expressed by many people on PE forums. I have experience with a few of them myself.
I am not sure about the causal link between BPFSL-differences and EQ, but in my case I would have always had at least 0.75”+ difference, up to 1” difference between BPEL and BPFSL. If I concentrate too much on stretching and length-work my EQ would plummit as a result. Very often when I only concentrate on length, my BPFSL will increase but my BPEL will remain the same. I will often have to throw in some girth-work and EQ-improving exercises in order for my BPEL gains to materialise.
I have read numerous accounts of guys with 10/10 EQ, not having this same issue with length-work; that they can concentrate on stretching, hanging, and so forth without it affecting their EQ and they have gained quite well. I have also read many accounts how guys with 10/10 EQ break up their routine with excess girth-work or limit their length-work and cannot gain length.
There definitely seems to be a close tie between these examples and what the theory is purporting. However, I am not sold on the theory as proof, I am simply pointing out that I have seen massive corollaries in some of the links.