Are there privacy advantages from SL using the Proton infrastructure?

I incorrectly assumed that email between Proton users, using SimpleLogin addresses, would maintain E2EE. It does not.
In spite of this, are there still privacy advantages from the fact that SL uses the Proton infrastructure? For instance, is there any reason that such emails would ever have to leave the Proton infrastructure to be delivered (e.g., Proton email User A via SL to Proton Email User B)?

Even if they had to leave, they use TLS, so no one could eavesdrop the connection. But yes, Proton controlling the entire infrastructure makes it more private since they control the servers it would pass through.