The question comes down to whether you should use the points from the same book as the datasheet, or use the datasheet from the codex with the points from the index. To me, it doesn't make any sense to be pulling points from a different book than the datasheet. If we look at say, Hellblasters, they gained new options with the codex, so clearly we need to pull their points values from the codex, since they don't exist in the index at all. It would make the most sense for that to carry over to any other unit that's being used from the C:SM book.
I guess a decent example would be: can a tactical sgt take melta bombs?
The C:SM errata added them as an option; AFAIK no similar change has been made to I:I1. By the WC article, we *can* use the C:SM tactical squad datasheet and so the MB option is available to us.
Though, as always, a gaming group where this is an issue probably has bigger ones to deal with.
No that wouldn't be a good example. Options on the Datasheet and point costs are something entirely different in 8th.