I think this comment is based on some confusion about how languages spread. Languages spread along with people, but while a local language may be replaced, the people are not generally replaced with the language. There may have been some genetic mixture, there may have been a time where they were conquered by them for a time, but there's no sense in which the people who wrote those works _were_ Yamnayan, any more than the Germans are. They wouldn't have a story about having a far away homeland because they wouldn't have had a far away homeland, and nobody would have remembered any previous language because that language had been replaced thousands of years before, and well before anybody started writing anything down. They gradually picked up the language of either invaders or their trading partners, just as has happened many other times in history.
Edited to add: there are basically no migration stories in _any_ indo-european mythological cycles or oral traditions. That's not evidence that there wasn't spread through, migration or invasion, but it does indicate that it was a gradual process that wouldn't have been particularly noticeable in any one life time.
Edited to add: there are basically no migration stories in _any_ indo-european mythological cycles or oral traditions. That's not evidence that there wasn't spread through, migration or invasion, but it does indicate that it was a gradual process that wouldn't have been particularly noticeable in any one life time.