Why boil mixture when soaking fruit in alcohol for churn-free ice cream
I see a couple of reasons for bringing the small amount of rum and the raisins to a boil.  First, this process will plump the raisins fairly quickly.  Secondly, much of the alcohol will evaporate.  Alcohol freezes at a much lower temperature than water.  So, alcohol additions to ice cream make the final product softer.  In this case, it would seem that given the no-churn technique, a little alcohol would be beneficial. So, you could leave them to soak without boiling, but it would take longer for your raisins to plump.  So, alcohol will change the final texture, but you might like the result.  It is possible to add too much alcohol.  Max Falkowitz of Serious Eats writes that adding more than 5-6 tablespoons in a quart of ice cream base would result in an ice cream that doesn't freeze enough to be scoopable. If you eliminate the boil step, you will want to make sure you don't over do it with the amount of rum.