Wow, you still have sumo fruits on your tree this last. I have never heard of anyone else with ripe sumo in September. Let us know if the inside flesh is dried up or is still good to eat.
These set *way* out of season, it isn't that they have been hanging on the tree all this time since winter. They started ripening in summer. I also have some much less mature fruits currently maturing at what I think is the normal schedule. The ones that are now yellow are becoming overripe, but the green-to-20% yellow are pretty good tasting though not as good as the usual crop. When cropping on a normal schedule they are best when full orange.
FruitGrower, I don't have a good understanding of citrus growing in tropical or near-tropical regions. I have read that citrus in Caribbean and India do not ripen to orange color and that cool weather is required for orange coloration, but I am not sure if that only applies to certain varieties or not.
As far as sweetening the fruit, again I am not confident in what is normal but I understand that some citrus, notably grapefruits, require high peak temperatures for some time to sweeten fully. I don't have details and I am not sure of the extent of it.