I consider the Roles video interview between Gamer_Zero and James Wyatt as official information. If you don't consider it official, stop claiming there are four roles, and that their names are defender, leader, striker, and controller, as that video is the only source that confirms all four, names them, and details what they do.
Since that video interview is official 4th Edition info, whatever is said therein, is also official 4th Edition info...
Roles Video said:
James Wyatt: ... We've narrowed down the functions that each role performs, and designed different classes to fill each role equally.
GamerZero: So, for example, you can have a cleric, which is great, or you can have a druid who is different from the cleric, but can fill that primary role of being the primary healer in the group.
JW: Just as good. Right.
Look at that, according to
James Wyatt, in the video interview, druids will be "just as good" at healing as clerics. Check. Official info. Thanks James. Unless James Wyatt isn't a good enough source for you.
PC Roles article said:
Unlike their 3e counterparts, every Leader class in the new edition is designed to provide their ally-benefits and healing powers without having to use so many of their own actions in the group-caretaker mode.
So, the best healers in the game, those with healing abilities prominently built into their class, by design, are the leaders. And that's according to the lead designer of 4th Edition. Hopefully that's a good enough source for you. (Paladins are not officially stated to be "just as good" at healing as the cleric so keep that one to yourself.)
Unless they spent all that time designing the roles, just to abandon them for the druid, the only way another class could be "just as good" at healing as a cleric is to be another leader class.