One foreign gene taking hold naturally per species per million years would not be ancient, rare, or unimportant. And it would also nothing like the rate GM is being used to introduce them into common food crops, to the point where it wouldn’t say much about it.
So if there isn’t actually have any real reason to believe it natural transfer does more than that - which I am disappointed to find - I will go back to calling it a specious argument as per above, and wondering why anyone would bother with it in place of the real evidence about safety.
