With all due respect, this is not what I'd call constructive feedback at all... And, it is offensive.

Adding "No offense" wouldn't make it any less offensive.

Your feedback is devoided of compassion and emotional intelligence.

"Honesty" is anything but a good thing when it doesn't add any value and when it's downgrading others to feel better about ourselves, it seems to me.

The author is secure enough not to get offended by your words and I'm glad for that, but it was hurtful and so off.

You didn't even answer his request about how he could better his craft. Zero plus-value.