It's still effectively possible for someone to only have 1 kid that had a child that had a child etc. Each link could have multiple children as long as none of those descendants had children that survived to the present day.

This becomes more likely if you include genocides that may have quipped out large swaths of someones descendants.

You also have rather insular groups where a few thousand people may be largely segregated from the rest the world. That's not going to be particularly stable over hundreds of years, but you only need 1.

Not genetically in that spot but doing family stuff discovered that I'm on a very out there part of my family tree (male decendent wise) four generations of only one son :-). It reminds me that improbable isn't a good defense.