I assume this conclusion was hard to make empirically in the days of slow travel -- one could not as we can today fly from above the equator to below it and observe the difference in weather and even if one could, it would be hard to attribute this to a single cause as weather is very changeable.
I further assume some ancient understood the thing about the Earth being tilted on its access but even to me (not an astronomer or even very sharp mentally) it is far from obvious that this would affect the seasons -- I do think that only if one accepts Heliocentrism that this phenomenon can be accounted for.
Is it then possible that no one in western Europe in, say, 1500 believed that seasons were reversed and could explain it. I do not know what Indians or Chinese knew about this.
EDIT: I realized that a ship sailing south enough would still know what date it was and people might well note that it was colder or warmer than the same month where they came from. They might attribute this to local weather but if the differences were pronounced enough, at different longitudes, someone might suggest that indeed seasons were reversed without understanding why.