See Anna E's answer and also note that if someone is selling a rum cake with enough rum to get you drunk - they would have to sell it with that notification and only to adults.
If you want boozy rum cake - make it yourself. Make Baba au Rhum - it soaks in a glaze that includes uncooked alcohol. Here's one recipe
https://www.foodnetwork.com/recipes/ina-garten/baba-au-rhum-recipe-2103678