Without doubt, landmines are a effective and cheap killing tool.

But they kill or cripple anyone. And the person placing them, usually does not remove them anymore.. so lots of one legged children in the end, as normally (rightfully!) war places see peace again. But with mines being cheap to deploy and removing them beeing very expensive they tend to stay where they are, until your childrens children walk over them as boyscouts.

And south corea had US backing as far as I know.

At the time the landmines were placed the US was at a stalemate with North Korea (and China) - and even the stalemate was pure luck as NATO forces were close to full defeat at many points after China entered the war. US backing did not make their survival a sure thing.

Another interpretation is, that the mines cemented that border.

Yes it did. But the alternative border would have been a unified Korea with the north’s leadership in charge. Whether that is worth the trade off depends on your politics I suppose.

Or a tight border for some years and than a reunion like in my country, germany?

(even though there were mines on that border, too, but in a limited way that there are no more traces of them today)

Anyway, I am still highly sceptical to the claim, that mines prevented a total communist korea. Debating about that is not really constructive. Do you have studies for your claim?