The recommendations that I have seen are that up to around 50 g of sugar per day (25 g of fructose) should be fine.
Most fresh fruits have about 10% of sugar, so you need to eat more than a pound or a half of kg to reach levels of sugar intake that could be unhealthy.
Only a few fruits have more sugar, above 15%, e.g. grapes, fresh figs or fresh dates. Even with those you must eat an amount over what would satiate most people, in order to get too much sugar.
On the other hand, if you eat one chocolate (many of which contain 60% or more sugar) or a few candies you are likely to have already exceeded an acceptable daily intake.
And still, that doew not mean you nmed to cut complex carbohydrates and go keto ... especially if argument is how people 300 years ago eate. It was not keto.
And it is not used that way by people who push keto and low carbohydrates diets on HN.
And again, it renders fat vs sugar dichotomy meaningless.
The context here is that major food group that was pretty significant source of calories - complex carbohydrates was skipped creating dichotomy between sugar and fat.