You also need to do the opposite experiment Have two people put their phones in the microwave (don't run it!) / turn them off, discuss swimming pools (or jacuzzis etc), and see if you suddenly start seeing ads for said thing. And then you would have to repeat this experiment several times to rule out outliers

Experiment design is important! I completely believe that this happened to your friends and I also don't think it means what you/they think it does

(That is: you need to completely isolate yourself; music practice room on a college campus where nobody is wearing a watch or phone and repeat the experiment. If it turns out that you still see ads for that thing, then the experiment didn't prove anything)