Not necessarily true, I think. It’s a mental barrier more, than a physical one. Tell someone you ran 100 m in 12 seconds and they go “meh”. Tell them you did a marathon and they go “i could not do that, awesome, how could you etc.” whereas an average person can do a marathon with a years worth of training and never achieve that sprint time.

It took for me 21 months of training to go from effectively zero to a half ironman and the biggest barrier was swimming, which is very technical in nature. I still can’t swim very well. I was in no shape at all and overweight. If you look at the speeds required to stay within ironman time limits, they are quite low and a person in a slightly better than average condition would be able to sustain them with relative ease. The barriers might be swimming technique (which one might have from training as a kid) and things like biking position and running technique but not necessarily fitness as such. In fact, I see quite a few people visually in much worse shape than I am (i.e. fatter) doing long IMs and beating me at shorter distances.