I agree that the info in those 2 links should probably be rewritten and it actually looks like catch edited the text in the second link to account for the extended 6 plateau boosts but forgot to adjust the numbers.
As far as the question/answer? 6 boosts at plateau in this case.
I treat plateau (day 281) as a rollover in itself. You lose pre-rollover boosts if not used and gain the plateau boosts. As long as your dot is level 70 on day 280, you will hit level 79 if you boost at the appropriate times. The rolling xp cap assures 2 things:
You will need to spend the same amount of flex to hit level 79, regardless of boosting on creation
Every level 79 player will hit end game with the same amount of total SP available to them. (Opposed to needing only a couple of boosts to hit level 79 when xp kept accumulating and some of those levels were natural with only 5 SPper level)