I don’t care what “they” said or where “they” said it (or who “they” is specifically for that matter). Unless it is in the Manual, it is not official. The Manual explicitly states that Co-op points are seeding points, and that all seeding points go into your seeding score. Highest Co-op bonus is tracked but not displayed, and is the first tiebreaker. Note that it’s the highest one, not the sum of them.
And you’re reversed on the Co-op bonus makeup. It’s not that the bonus is based on your seeding points, it’s that the bonus is extra seeding points.