Didn't the bronce age end like three thousand years ago? It's my understanding that by that time the Indo-Europeans were already well established. Besides while they contributed a large fraction of the ancestry of contemporary Europeans a significant fraction of the ancestry is still from the middle eastern farmers which conquered Europe before them. Even if you only look at the Y chromosome, there is still quite a bit none R1 floating around.

It's true that they were replaced, almost completely in northern Europe. But the claim of TFA is that these skeletons date from after this Indo-European / Yamnaya arrival, and hence that it disproves the theory that lactase persistence arrived with them.

Linguistically, I think we know they had cows but not agriculture, before migrating to Europe & India. (IIRC "cow" comes from PIE?) Which gave credence to the idea that they may have been lactose-tolerant. TFA is one study, so we will presumably eventually see whether this holds up with more data.

Do you know where modern day Europeans came from? Who were these invaders?

A variable ancestry of post-ice age hunter-gatherers, Neolithic Anatolian farmers and steppe invaders (ie. Indo-Europeans). The ratio varies depending on areas, but that mix is specifically European.

Indo Europeans from pontic steppe and farmers from Anatolia. Northern europeans have more ancestry in the prior.

Possibly the celts and prior Indo-Europeans?

https://en.wikipedia.org/wiki/Celts

You can find answers to these questions in recent David Reich’s book.