I mean, they could add "for this event" at the end of that sentence, but the meaning still carries and they'd probably rather have one message.
Your datacrons will be available next week. Yes, you add them to use with a specific squad at the battle screen. But what you were told above is that your whole roster locks in for the week when you sign up. If you swap mods, raise gear, etc., those won't be reflected in the current week.
It may be helpful for you to look up a guide on datacrons to get all the ins and outs. For example, you can use all the stats of a datacron (crit damage, etc.) even if no one on the team qualfies for the level 3, 6, or 9 bonuses.