Can you thicken a pudding just by reducing?
This will only thicken by reduction if you leave it on for a really long time. At least 8 hours of simmering, but better to use more than 16, and then you'll get a flavored kaymak. 

The way it thickened is much more likely to have been raspberry pectin. Many berries have sufficient pectin to thicken when they happen to be used within the optimal sugar and pH range. The calcium in the cream will also activate the second type of pectin. 

I have had raspberry juice make a jelly by itself, and I have made a recipe which thickens a gooseberry-cream mix, relying on the gooseberry pectin. I haven't made a cream based raspberry thickened dessert, but it's very likely to work. 

You can use the gooseberry recipe to get an approximate range for the sugar amount involved, http://www.bbcgoodfood.com/recipes/1415683/gooseberry-cream-and-elderflower-jelly-pots.