How do I stop a private compliment becoming a public book recommendation?

My friend sent me a square graphic with my name under the words, 'I couldn't put it down.' Above it was the cover of her first novel. She asked, 'Okay to post this tonight?'

I'd bought the book and messaged her that sentence after reading it on a long train ride. I meant that I wanted to find out what happened. I also found the final reveal confusing, but hadn't raised that because she'd asked whether I'd enjoyed the trip, not for a review of her novel.

Now she wants to use my message to promote it. I'm pleased she's finally published, and the sentence isn't fabricated. But next to the cover, with my full name, it feels like a recommendation to people who know my taste in books. I wouldn't recommend it without mentioning my reservations.

She hasn't posted anything yet. I'd be comfortable with a photo of us at her launch, but not this quote graphic. Can I decline without making the refusal a surprise critique of the book? I don't want to give her a glowing alternative I don't mean just to make the conversation easier.