The advantage to filing jointly is actually when the couple makes sufficiently different income, and the penalty is actually when they are close in income.
I was simplifying, and your simplification is more accurate, but neither simplification quite captures it. Both aspects play a role, with the amount the couple makes essentially scaling the benefit or detriment. If a couple makes the exact same amount but it's a low amount, there's zero detriment, but if it's a high amount there's a significant detriment. If they make very different amounts and it's a low amount, there's a significant benefit, but the size of that benefit decreases and can disappear entirely as they make more.
