I agree with you, but I think the parent was saying that even controlling for one's ability to successfully do a thing, some things (i.e. those that are more strenuous or otherwise require more commitment) take longer to solidify as habits, which makes sense if we think of habit-forming as environmental adaptation. One doesn't adapt to massive environmental changes after just a couple of weeks.

Ironically, long distance running (10+ miles) is easier to do before breakfast, as an empty stomach is useful--digestion stops anyway as soon as your body is in need of some extra red cells to carry more oxygen. A trained person has enough ATP reserves for about 90 minutes at threshold intensity, and anyway the whole process of ingesting, digesting, and metabolising a regular meal takes many hours (around twelve in my case), so having breakfast before a run is purely a psychological boost--bar quickly metabolising food, such as banana or sport gels, which you do ingest during the run anyway.

All this of course depends on the individual's physiology and psychology.

For me, I tend to run either before breakfast or in the wee hours of the morning (one/two o'clock) before going to bed.

Mind, I'm an ultra runner, where being hungry and sleep deprived for days is the name of the game, so I've overcome that particular psychological barrier.