I was wrong actually, Tassard was right.
They haven't changed the rule. They just made it really confusing.
Articles 17.07, 17.17 and 17.18 make it confusing
Here is article 17.07 from UEFA regulations :
17.07 Excluding the three rounds of the UEFA Intertoto Cup and the UEFA Super
Cup, and subject to paragraph 17.18 below, a player may not play UEFA
club competition matches for more than one competing club in the course of
the same season. A substitute player who is not fielded is entitled to play for
another club competing in the same season’s UEFA club competitions,
provided that he is registered with the UEFA administration in accordance
with the present Regulations.
And this is article 17.17/ 17.18:
17.17 For all matches from the start of the first knockout round, a club may register
a maximum of three new eligible players for the remaining matches in the
current competition. Such registration must be completed by
1 February 2009 at the latest. This deadline cannot be extended.
17.18 One player from the above quota of three who has played UEFA club
competition matches for another competing club in the current season may
exceptionally be registered, provided that the player has not been fielded:
− in the same competition for another club
− for another club that is currently in the same competition.
This means that a player who has been fielded in another competition for another club, is eligible to play in another competition, which is the case with Huntelaar and Real.