I'm saying that there is no way the "news" would KNOW if "all" the ATF people stayed away or not---it's not in their power to know that. It's clearly a case of some guy saying that and an irresponsible (or lazy) reporter passed it along as truth.
It's very possible to know whether every ATF field agent was absent or present that day, if you go down the roster and account for each individual. It's time consuming and possibly difficult, but not impossible.
As to whether it's "clearly a case of some guy saying that and an irresponsible (or lazy) reporter, etc., etc.," I have no idea why you'd draw that conclusion. Charles Key's commission found that ATF agents were absent from the building. Whether that means some, most, or all is unclear from the sources I've cited, but my memory of the reports at the time is quite clear.
Now, it may be that both Mr. Key and myself are paranoid kooks. If we're going to be rigorous, it's best not to disregard that possibility. =]