Looks like they did “Brakerski-Fan-Vercauteren (BFV) HE scheme, which supports homomorphic operations that are well suited for computation (such as dot products or cosine similarity) on embedding vectors that are common to ML workflows” namely they use a scheme that is both secure and efficient specifically for the kind of compute they do here. https://machinelearning.apple.com/research/homomorphic-encryption
Looks like they did “Brakerski-Fan-Vercauteren (BFV) HE scheme, which supports homomorphic operations that are well suited for computation (such as dot products or cosine similarity) on embedding vectors that are common to ML workflows” namely they use a scheme that is both secure and efficient specifically for the kind of compute they do here. https://machinelearning.apple.com/research/homomorphic-encryption