The tendency toward cultural chauvinism and exceptionalism is hardly confined to Japan. I recall when living in rural France the dismal failure of an English couple to convince people that their restaurant could serve authentic local dishes.
Non-Japanese who dedicate themselves to mastering a Japanese art or cultural practice, who have the courage, or temerity, to practice it professionally, can expect a certain degree of skepticism — even the odd snarky remark from those around them. You would suppose that the practice of gardening in Japan, ostensibly borderless, would be the exception.
Foreigner visitors have taken an interest in Japanese gardens for a very long time. Luis Frois (1532-97), a Portuguese Jesuit and chronicler, gave early accounts, largely favorable, of the gardens he visited in Kyoto and Nara. Engelbert Kaempfer (1651-1716), a German physician, and Carl Peter Thunberg (1743-1828), a Swedish naturalist, provided detailed critiques of gardens and flora. British horticultural magazines dating from as early as 1800 contained many in-depth articles detailing the elements of a Japanese garden.