Maybe it does, maybe it doesn't. But when you pull out from the kerb in a motor vehicle, do you have a look behind, or just wind down the window and listen? Tell the truth.
The fact is that wearing headphones does affect your ability to hear other things. Here's one or two other facts: 1. Without looking, hearing is not the only way to detect an approaching motor vehicle. You can feel the vehicle approaching through your bicycle frame and body.
2. No experienced road user ever only used their hearing to make decisions about their next move. An experienced cyclist can easily compensate by being even more vigilant with their visual senses, depending on the situation they are in, sacrificing practically no safety at all.

I've cycled many 10's of 1000's of kilometres using IEM's, in all types of speed zones, and have never had a motorised vehicle overtake me, without my knowing of it's approach. The largest and most dangerous vehicles I routinely detect from well over 100m behind me, when travelling at highway speeds.

I have had cyclists overtake me that I never knew were approaching. By holding a straight line on the road they were able to overtake in safety.

People need to stop claiming that you can't safely cycle with earphones, because that just.isn't.true.