To be fair, quite a lot of people use cedar shavings (mixed with pine, or all cedar) in coops and nothing detectably bad happens. Indeed, a lot of giant commercial broiler farms use cedar as bedding, and they have *very strong* incentive to minimize-minimize-MINIMIZE mortality as much as possible.
I am not saying there can't BE problems from using cedar -- IMHO, it is likely to depend on the particular batch of shavings you've got, and on how well ventilated the coop is -- but I think it is demonstrably inaccurate to say that you *can't* use cedar safely.
I wouldn't, myself, but that doesn't mean it can't be done.
Just sayin',
Pat