It's not a myth. You're both kinda right:

1) The fruit that we eat today is hyper-sugarized and available 24/7 all the time. Eg European ancestors NEVER had access to this amount of fruit. Apples, bananas, (water)melons, sweet grapes, ...these things didn't exist 150 years ago in the form we know today. People eat these sugar bombs all the time.

2) There does exist high sugar natural fruit.

Even your modern European cousins don’t have quite the same level of access to super-sugary fruits year-round, or at least we see some blatant price differences.

Cherries right now (mid-July) in northern Bavaria: $3/lb at the grocery store, cheaper if you buy direct from farmers just outside the towns. Various small plums are free for the taking from trees along trails and rivers. I’m planning some serious jam-making over the next few weekends.

Cherries in January: if available at all, approximately $20/lb, import from South Africa or Chile. Plums are also available, about $8/lb, but not very flavorful.