Michael Jon Carter, aka Booster Gold, is from the 25th century. That is, conservatively speaking, at least twelve generations removed from today. Booster should have somewhere in the neighborhood of 10,000 ancestors living today, of various ages. It is likely that number could be upwards of 50,000.
So the real coincidence here isn't in locating an ancestor of Booster's, it is in stumbling across Booster's (presumably) direct paternal ancestor. A Carter. Of which there are probably, at most, three: Daniel Carter, his dad, and his granddad.
If we assume that Daniel is twelve generations removed from Booster, then there should be 2047 other male ancestors of the same generation. Skeets just happened to find the one who contributed his surname and his Y chromosome. That's the real coincidence; not finding one of tens of thousands of direct ancestors.
This character device of the direct male ancestor has been seen in comics any number of times before, but the use that perhaps irritated me the most was in one of my favorite comics, Starman. During the "DC One Million" crossover, we were introduced to Jack Knight's descendent from the 853rd century, Farris Knight. 85,000 years removed, well upwards of 2000 generations, and it's still a direct male descendant who's carrying on the family business.