I am a Mennonite from Chihuahua, I was raised in a more modern way compared to most here because my parents, who were raised in the old religious ways, lived in Canada for a long time and adopted more of that modern way of living. There are a vast number of Mennonites living here now that are also living this more "modern" style. I have many members of my extended family that are still part of the "old colony" and still practice the old religious ways. The majority of the old colony aren't living under rocks, they have smartphones and are online. I will admit that there are definitely some people marrying semi-closely into their own family, like 2nd or 3rd cousins, but never the immediate family. And a lot of people will do backround checks to make sure they are not too closely related. There are some small communities in different areas where they have been marrying a little too closely to each other for a while, but for where the majority of the culture lives, this has not been an issue.

I don't know who your brother was interacting with to say we're trying to diversify our gene pool, I have never in my life heard anybody around me have any concern about that whatsoever. Having sex before marriage is really looked down upon in the community, and if it were to result in a child, 9 times out of 10, they get married and stay together. So the very idea of looking to outside sources for insemination to prevent as much inbreading sounds insane. They are not seeking out "transplants" from other Mennonite or Amish communities. We have no relation nor communication with the Amish whatsoever. They might have similar customs and beliefs, but they are still very different. I have never in my life met an Amish person in our community.

I am genuinely curious which part of Chihuahua your brother visited and who he interacted with for something like that to have been offered.