Working through it, it makes sense, but we're basically saying the same amount, you just said it while including detail for the entire raid (My own left out healers and the Red Mage: Your target was 8, mine was 5)
However, .75% of 20,000 is still approximately 150 that belongs to the Red Mage, which must be added to their bottom line. That's why it's fairgame to consider. Embolden is damage, not utility.
Using those figures on 95th percentile bosses, where Black Mages range from 4200-4700, and Red Mages from 4200-4400, adding 150 of that to their rating puts them on similar levels, to the point where there's no true advantage of the Black Mage unless we move into caster heavy set ups, which don't really have the support that physical do. It's worth noting that going down in percentiles shows the natural gap closing before applying the 'Embolden' tax.
I need to be clear that I'm fine with damage of jobs being comparable all around, with the minimum being within their roles. But in the scenarios where this is the case, each Job should be bringing some form of non-damage/ mitigation utility. Just giving 'more damage' is a lazy and uninteresting way to fix things, because that just shifts around who's useful when.