This is true, (see my second to last paragraph) but ... that is not as widely known.

Most people just think 'Idk, Irish people like potatoes'.

Most people think it is just in the realm of dietary preferences and cuisine and don't know why.

Even though there is a fairly direct equivalence if you actually know the history of the stereotype, most people don't.

Thus you are perceived as overreacting if you jump right to genocide.

To attempt an analogy:

It's like if a boomer tells a millennial or gen z to pull themselves up by their bootstraps, and they point out that the original meaning of that phrase was meant to encapsulate the literal physical impossibility of doing so as analogous to making a decent living for yourself in an economy where very very few people have the means/opportunity/connections/dumb luck to earn more than basically a subsistence wage.

The boomer just gets offended or bewildered because they didn't know that, or they don't believe it, and they're too arrogant to admit they have no real, useful advice, and that all they have to offer is infantilization.

Their ignorance (and inability to admit their knowledge is flawed) allows them to keep using an offensive phrase and not see this as offensive.