Earlier quoted context omitted.
The obvious difference is that by sending your photos with Signal, you are doing it willingly. You let it encrypt and decrypt willingly. You decide who gets it. Here, Apple does that for you.
Your ISP, a bunch of routers and switches and the servers run by Signal can also see your encrypted photo. You don’t really get to decide who sees the encrypted photo. You do get to decide which photo you encrypt and send though.
Everybody knows that there is a network infrastructure where your content flows through. You willingly accept that as a connected device user because it is necessary to be connected.
What Apple did is not necessary and users don't know about it.