Another unsubstantiated claim. The Japanese may have high life expectancy, but they have
one of the world's highest rates of gastric cancer despite their healthy cuisine. It's probably genetic since Mongolia, Korea and China are the other countries with extremely high rates. The fact that you could suggest that the Japanese or the Poles are inbred in any way means that you do not have a clue what inbreeding is. It doesn't mean that people don't intermarry much with foreigners or outsiders! It means that people tend to frequently marry first or second cousins. It's not something Japanese people do more than Westerners, and probably less since Japan has such a high population density and urbanisation, which increases the chances of meeting and marrying completely unrelated people.
If you didn't mean that Japanese people were inbred, but ethnically homogeneous, this is also an illusion based on the linguistic and cultural homogeneity. Japan has a population close to 130 million inhabitants, almost exactly the same as the whole of Africa around 1900! Even in 1900, Japan had 45 million inhabitants, so about three times less than Africa, simply because Japan, like China, has had a huge population for many centuries. The higher the historical population, the more mutations and the less homogeneity. But even if you look at prehistoric ancestry, the Japanese are actually
a blend of two completely unrelated populations: the Sino-Korean derived Yayoi farmers (Y-haplogroups N and O), who started colonising the archipelago 2500 years ago, and the aboriginal Jomon hunter-gatherers (Y-haplogroups C and D).
Nowadays Southwest Japanese, like in Kyushu, are about 2/3 Yayoi and 1/3 Jomon, while Northeast Japanese have the opposite proportion, meaning that in terms of ancient admixtures northern and southern Japanese are more different than the French are from the Germans. It may be harder to notice these differences physically. But looks aren't a good indicator of genetic differences. A Siberian may be confused with a Chinese or even a Vietnamese, but that doesn't mean that they are closely related genetically. Likewise if you travel around the UK and look at the locals (excluding recent immigrants) you will see people with extremely different looks, a whole range of different pigmentations, with red, light blond, dark blond, reddish blond, light and dark brown and black hair, straight, wavy and curly hair, people who are very short and others who are very tall, some who have very big noses and other very small ones, round heads and long heads... Yet they are all fairly close genetically, and in fact closer with each other than with anybody from say Denmark or France, even though some British may look very Danish and others very French. That being said, I can usually tell a Japanese from Kyushu apart from someone from Tohoku, because I have lived in Japan and analysed their phenotypes.