I can't answer for him but I would like to add what I liked about Arrival that I didn't like (on first viewing at least) about Annihilation.
*Arrival Spoilers*
I don't like labeling shyt "deep profound" because one mans deep question is another mans obvious answer, so I don't wanna get into pissing matches about what is deep or not. But the question that Arrival presented to me that resonated emotionally was "Is it better to love and have lost than to have never loved at all?" It's a very old question, something I've been intrigued by since high school after my first breakup. And the way it cleverly stitched that question into the emotional center of a hard sci-fi alien contact film was something I was hit hard by at the end.
It recontexualizes all of the "flash back" scenes prior to it, and suddenly Amy Adams subdued interactions with her daughter make a lot more sense, and is pretty heartbreaking in hindsight. She knew that she was going to fall in love, have a daughter, watch that daughter deteriorate and die, and lose her relationship because of it, but she still chooses to live through it. Arrival resonated emotionally with me because of that. The structure of the film was rewarding. And it needed that structure to work.
While I think I followed all of the flashbacks and cerebral questions and themes about depression and suicide in Annihilation, viscerally, none of it resonated with me emotionally. I watched it detached, intrigued, but not really invested, never really inserting my own emotions and thoughts into any of the characters.
I've been depressed, I've been sick, I've been suicidal, but for some reason those themes still never made me feel anything while watching it be explored with those characters. And I never really felt like the out of sequence structure gave me anything rewarding, outside of a vague sense of mystery and suspense. But mystery and suspense was already there, it didn't need that structure.
I felt investment in Amy Adams final decision, in as much as she can make decisions in a closed time loop. For that reason, Arrival worked on a level for me that Annihilation never really reached.The flashbacks coalesced into something that worked plot-wise and thematically. Not really sure it did in Annihilation.
Maybe that'll change on subsequent viewings because Annihilation does have a lot to offer in visuals and concepts. I just hope when I watch it again it'll actually make me feel something (other than dread, it hit the ball out of the park when it came to that.)