Back then I understood that an operation like SUM would be able to compute the sum of a list of numbers where each number was encrypted. The way the encryption worked made it possible to add all the values together without decrypting them, and the result ended up being encrypted too in such a way that the owner could decrypt it and have a number with a certain known accuracy.
If Apple is using homomorphic correctly then there should be no way for them to see the data they get from your phone. The other things they mention in the post as ways to prevent leaking of other information through metadata or a side channel.
The fact that this feature was enabled by default isn’t exactly great. Definitely should have been something that the user should have been asked if they wanted to enable after upgrading.