You can mitigate tin whiskers with a conformal coat application. It's not going to be as good as lead solder but it is an acceptable method when you have no other option.

Another possibility is that these are modified CM4 boards where parts were removed and resoldered using lead solder. The Broadcom chip is really the most critical component as you cannot buy them off the shelf but everything else could be purchased new and installed onto a stripped board or a brand new one made using the CM4 design files using lead solder.