By swapping the DI buff, you only have 2 people with the 9% increased DOT damage. If the two Warlock's put their DI on other targets, there would be 4 people in the raid with 9% increased DOT damage. The overall gain in doing so is shown in the original post. There is no way that increasing the two Warlock's up times from what may be a difference of ~15% could outweigh the overall increase from other raid targets.
If you're looking purely at up times and personal DPS gain, then your point would be correct, but that reasoning is very counter-intuitive to raiding.