If true (and Twitter says it isn't), it doesn't really surprise me.
Most brands struggle to get any sort of meaningful engagement with their posts on Twitter and that's usually for two reasons:
1. Most people don't like brands trying to engage with us on Social Media - we would rather engage with real people
2. Instagram's user experience is based round images which most people prefer to look at than text. Yes, you can add images and gifs to tweets, but text is still what you see first.
Imagery in Social Media is extremely important - and even on sites like LinkedIn, posts with images do significantly better for engagement levels. Also, people who add images throughout their personal profile tend to get decent levels of views. When LinkedIn first added their Professional Portfolio tool, I saw a massive jump in profile views immediately after I started adding more pictures to my profile.
Instagram is not for everyone, but the fact that it has imagery at its core will always make it compelling to advertisers.
Instagram has beaten Twitter as the most popular platform for advertisers and their social media campaigns for the first time, according to a new survey that has been rejected by Twitter.