Solid fruit is not the problem, juice is. You wouldn't eat 10 oranges at once, but you can drink 10 oranges worth of sugar in a glass of orange juice.

Would you consider a fruit smoothy a problem if it has no added sugars?

Total sugars is the relevant metric, although added sugar serves as a good proxy if you’re comparing nutritional labels.

A fruit smoothie that keeps all the fiber and hence is more filling and hence causes you to eat less sugar by way of eating less fruit overall is fine. A smoothie where you filter out the fiber or otherwise eat excess amounts of sugar would not be.