Responding to ”I’m a pure Japanese”

Am I the only one who gets triggered when I hear Japanese people proudly claim they are "pure Japanese" when talking about their origins?
I’ve heard it so many times in expressions like ''100%日本人'' or ''純ジャパ'', and every time I cringe. I’ve also seen some nationalists claim that Japanese are a monoethnic people. That is historically so wrong, and to me this idea of a "pure race" is nothing more than a myth and a vector of racial discrimination.

Is it really necessary to remind people that Japanese descend primarily from two ancient groups, the Jomon and Yayoi? Later, Japan was heavily influenced and connected with Chinese and Korean peoples, without whom it wouldn’t have developed rice agriculture, a writing system, Buddhism, Confucianism, metallurgy, architecture, etc. as we know them today.
Not to mention the Ainu and Ryukyuan peoples, who are part of Japan but are often overlooked for the sake of maintaining the idea of a "single race''.

Even in more recent centuries, the Japanese Empire colonized Taiwan, Korea, Manchuria among others, and many people from those regions came to Japan for labor and stayed (for example, Zainichi Koreans).
On top of that, many Asian immigrants changed their names to Japanese ones (I have a Chinese colleague who recently got Japanese nationality and did the same), making the traces of immigration blurred or erased.

To me, talking about a population being genetically "pure" is total historical nonsense and tied to nationalist or racial myths. Such ideas were promoted from Meiji Era and later intensified during Japan’s imperial expansion and WWII (comparable to the racial purity myth of the Third Reich), and are still echoed today in claims like “I’m pure Japanese.”
Of course Japan is far from being the sole case, but honestly, I don’t want to live in a world where this ideology of racial purity thrives again.

TL;DR
The idea of racial purity is a nationalist myth. Japanese culture and people are the product of centuries of mixing and outside influence, more or less like any other country.

So recently I’ve been thinking about how to respond kindly when someone introduces themselves as "pure Japanese," without being too harsh or lecturing. Any thoughts or ideas?