Many likely have strong points of view developed over the year. I doubt these are Kerry people who are leaking. That was not the case in his campaign, on the SFRC, or in the Senate.
Here, they seem to fault the WH, not Kerry, for not giving State talking points. This sounds like Kerry and his inner team scrambled to create talking points for the State Department spokespeople to use - something that had to be useful as Congressmen came out of the meeting and reporters sought statements from the State Department. Kerry was involved on this issue at the NATO meetings on Tuesday and (I think) Wednesday - so it may be that the WH knew that he and Hagel were already on message. As Hagel, Kerry and Obama all spoke with the same cautionary tone and in fact used phrases directly from that letter - this was clearly well coordinated between the three.
The reactions of Democrats and Republicans to Kerry's closed door briefings seemed positive as to the quality of the briefing they got from Kerry. It seems Kerry gave them an intensive, serious briefing. The reaction I heard was that Kerry spoke to them outlining options and the description I heard from a Republican was "powerful".
Looking just at that part, I think those officials missed the forest for the trees. Kerry had just returned from Brussels, where this issue was addressed, had obviously been in contact with the WH and was well prepared for the briefing -- and he scrambled to ensure the Department had talking points. Maybe Obama trusted him?