I don't really see how it can be true tbh, every human (and mammal for that matter as far as I'm aware) has a 'fight or flight' reflex and one of the main things that happens is a change in heartrate. This can happen because of stress, or fright, or excitement and is caused by adrenalin or cortisol changes in the body making the heart pump faster. We all know that when we're relaxed our heart rate is slower. After exercise or excitement it's much faster.
I don't really know why this would be different for the bubs, they're practicing everything in there!! They're probably relaxed most of the time, which is when their heartrate is slower, but then there are a lot of exciting things going on and then their heartrate goes up.
Seems like common sense to me? Don't want to argue obviously, but there are just so many myths out there I'd feel silly for saying this one had any kind of likelihood of being true (even in labour, which every woman and baby experiences differently with higher or lower levels of stress and excitement).