I'd really like to get my hands on this paper.
Unfortunately, its locked up behind A&A's paywall.
The statement about Earth's orbit becoming unstable, could also be interpreted to have nothing to do with Ceres' and Vesta's gravitational influences .. the article is not particularly clear on this. (Once again .. I recommend that the journo article be taken with a 'grain of salt').
I notice this 'Laskar' guy has been working on orbit variations throughout the solar system for quite a while. Some of his past papers are on arXiv.
Cheers