Your posting seems a bit contradictory on first read:
From YZD:"If emphasis was all on chan Si jing, then Yang style would be the same as the Chen style."
From Yang Lu Chan:
"...There is only one school of tai chi chuan: there are not two methods."
Until you read the word "all" in YZD's statement. That helps clarify things a bit, I think.
Silk Reeling is no more the entire art of the Chen family than Peng energy is the entire art of the Yang family.
I have had many lessons from Yang family instructors that encompassed many other aspects of the art then simply Peng energy.
The Yang Family Tai Chi Chuan is not all Peng, any more than it is only "open, rounded, and extended". It is a lot of things all rolled into one.
ALL of the emphasis in Chen style is not on Silk Reeling either.
I have never heard a Chen stylist claim that Silk Reeling, or dantien rotation, or any other single thing, is definitively their entire art.
The Chen stylists I have been privileged to work with have taught me many different things, silk reeling and dantien rotation being only two of them.
Their art is also comprised of many different things:
They have a Silk Reeling form.
And a Fa Jin form.
And a Pa Chui form.
A fast form.
A slow form.
A form that varies the speed.
They seem to use forms that are aimed at teaching one single aspect of their art at a time, then forms that incorporate all of them together.
It seems a bit much to those of us who practice the other family styles, which mostly have one form and then you learn the various aspects and incorporate them all into that.
But it seems to work for them, so I certainly can't argue with their approach.
Nor would I argue with the Grand Master of the Yang Family. I hope it is now clear that I am not doing so.
I am merely stating that doing something but calling it by a different name doesn't make it different. It's still the same thing no matter what you call it.
If I'm doing Yang Family form and incorporating the Essentials and Principles that it teaches and a Chen Family member points at me and says, "You're clearly doing Silk Reeling" then in my mind we're doing the same thing no matter what the Yang Family calls it.
That's no insult to the Yang Family.
It's the highest compliment I can pay them!
One Family. One Art!
Why should we be surprised when we see the other named styles doing the same things we're doing?
All I'm saying is that everyone can call these aspects anything they'd like. Just as long as they're doing them that's the important thing.
Which always make me think of this...
One of the first things I ever learned when taking classes with Si Kung Eddie Wu was this paraphrasing of how I remember his saying very much the same thing we're discussing here:
There is only one Tai Chi Chuan.
There are five paths you can take to reach it.
It doesn't matter which path you take, they all lead to the same place.
I'm sure I'm messing that up a bit, Eddie clearly says it better than I ever could remember. But I think that's fairly close.
To be clear I do not put all of my emphasis on any one thing; not Silk Reeling, not the 10 Essentials, not dantien rotation, rooting, not "open, rounded, and extended", not Peng, etc. Not when I practicing Yang style or when practicing Wu (Chien Chuan) either.
I pay attention to all these things, then nothing.
Meaning I pay attention to each of these things, sometimes only one, sometimes many together, to the best of my ability when I am training.
Then I do my best not to think about any of them when I am doing.
Does that make sense?
I don't feel it's good to get caught up in any one thing too much. To do so seems like it would just bog me down.
It takes all of these things, and then ten thousand more, to learn and use this art.
I try to learn as many of them as I can to the best of my ability.
Sometimes my ability with certain aspects is good.
Sometimes not so good.
All I can do is keep trying.
I have had very good teachers. I trust them implicitly.
Si Kung Eddie Wu was my first teacher of this art, a truly amazing teacher from whom I learned the basics. I still wish I'd have listened more and been a better student. Alas, I was young and didn't see the gold he was pushing into my pockets until much later.
Grand Master Yang Jun is now my Master and teacher. He is one of the best teachers I have ever met, he has never steered me in the wrong direction and has helped me to truly understand much of what I thought I knew but really did not.
Neither of them, not by word or omission, ever tried to dissuade me from learning from any of the other family styles.
In fact, both have flat out told me to learn everything I can from them. Many times.
So how could I not do so?
Master Yang has now held two Symposiums on Tai Chi Chuan. I have been privileged to attend both of them.
There we learned that, just as Yang Lu Chan states in your quote, "There is only one school of tai chi chuan: there are not two methods."
Which, at least to me, means that we're all doing the same things.
What we call them makes no difference.
Let's just do them and not worry too much about the names of things.