The source is here: https://berkeleyzencenter.org/2015/05/3 ... uki-roshi/For many of you, Zen is some special teaching. But for us, Zen is Buddhism and not a special teaching distinct from the other schools of Buddhism. So, if you ask me to talk about our teaching, I will talk mostly about the teaching of Buddhism, which started with Buddha and was developed by various teachers in India, China and Japan.
(non-mangled url: berkeleyzencenter.org/2015/05/30/from-a-lecture-by-suzuki-roshi/)
I wish there was some more context provided, especially: who does Roshi intend when he says "For many of you", and who is he referring to when he says "But for us"?
Informally, one comes across the kind of talk that Roshi is referring to all the time, that is, people who blithely proclaim that Zen has little or nothing to do with Buddhism. But it is less common to find people making this case in any kind of systematic way. To do so would require answering questions such as: (1) Has Zen always been separate from Buddhism? And, if so, was (the Buddhist priest) Bodhidharma aware of this separation? How about (the Buddhis priest) Hui Neng, or (the Buddhist priest) Lin Chi? Etc. And: (2) If Zen started out as Buddhist but is now separate (or at least separable?) from Buddhism, when did this transition occur?
I have found one source that addresses the issue a little more fulsomely than Suzuki Roshi:
This is from History of the Soto Zen School by T. Griffith Foulk (link: https://terebess.hu/english/zenschool.html/)[F]rankly, I think that many people involved with Zen in the West are confused about the relationship between "Zen" and "Buddhism." In general, we are too quick to proclaim the independence and uniqueness of the former and all too ignorant of the ways in which it has been embedded in the latter in East Asian cultures. We imagine that Zen is somehow a complete doctrinal, ethical, and spiritual system, and do not avail ourselves of the broader Buddhist resources -- scriptural, ritual, and institutional -- that Zen monks in Japan have always taken for granted. In fairness, it must be said that this "tunnel vision" that afflicts Western Zen is largely a product of modern Japanese Zen historiography, which (for social and historical reasons of its own) has tended to stress the "independence" and "purity" of the Zen school at certain times, such as the "golden age" of the T'ang dynasty patriarchs and that of the founder Dogen.
(non-mangled url: terebess.hu/english/zenschool.html)
A little further on from the part quoted above, Foulk very helpfully refers to Dogen's essay "Butsudo" in the Shobogenzo, where we find the following:
Personally I think Dogen might be overstating things a bit to make his point. Obviously the point is not what things are called, but rather what they are. To call something "Zen", it seems to me, is not the real problem. The problem is when this "Zen" is claimed to be anything other than, or separated (or separable) from "the great truth that is authentically transmitted from buddha to buddha", that is, "Buddhism".The Seven Buddhas’ and twenty-eight patriarchs’ experience of
the truth should not necessarily be limited to dhyana. Therefore the master of the past
says, “Dhyana is only one of many practices; how could it be all there was to the Saint?” This
master of the past has seen a little of people and has entered the inner sanctum of the
ancestral patriarchs, and so he has these words. Throughout the great kingdom of Sung
these days [such a person] might be difficult to find and might hardly exist at all. Even
if [the important thing is] dhyana we should never use the name “Zen Sect.” Still more,
dhyana is never the whole importance of the Buddha-Dharma. Those who, nevertheless,
willfully call the great truth that is authentically transmitted from buddha to buddha
“the Zen Sect” have never seen the Buddha’s truth even in a dream, have never
heard it even in a dream, and have never received its transmission even in a dream. Do
not concede that the Buddha-Dharma might even exist among people who claim to be
“the Zen Sect.” Who has invented the name “Zen Sect”? None of the buddhas and ancestral
masters has ever used the name “Zen Sect.” Remember, the name “Zen Sect” has been devised
by demons and devils. People who have called themselves a name used by demons and devils
may themselves be a band of demons; they are not the children and grandchildren of the Buddhist
patriarchs.