Before going too far with this hypothesis, its worth considering that general conscription doesn't mean that everyone has combat experience in Gaza - there are a lot of people whose military service involves office work, or patrolling quiet borders, etc. It may be that this sperm bank has a high proportion of combat veterans because getting into a top combat unit is something that many of the best male Israeli high school students aspire to, but in general, a lot of people serve in the IDF in positions that wouldn't expose them to DU.
This is especially true of older people doing their mandatory reserve service. My understanding is that if you have a particular skill, the IDF will find a way to use that. Actors get reserve duty entertaining soldiers, professors get reserve duty in the IDF education department, etc.