Ah, you have the book, I don’t! Ok, summoning still available, but not natively in the DG codex. So as you say, reducing repeated rules text. But also, makes it easier to completely change how it works if they so choose, as only the one place will have the rules.
Edit: @LansXero completely agree with the new army building rules making things more difficult for newcomers. Especially given the new Combat Patrol for DG has 30 poxwalkers, which have to be run as a single unit, given the additional restriction on not having more units of them than plague marines. I like them flavourfully, but being so tight has its own issues.
Regarding uniqueness, I feel the new contagions rules do a lot of work? Mass debuff effects of that ilk feels unusual.