I don't know how to estimate their reckless from the outside, but it is simple fact that however sensitive, they only had one waveband and so no real way of estimating the foreground contamination. I think it is also well established that the foreground estimation was made by looking closely at pictures from a presentation showing preliminary Planck skymaps of the dust emission, and then building a foreground model from that.
They also put out this adorable video as part of the announcement, maybe not the greatest show of caution ever: https://www.youtube.com/watch?v=ZlfIVEy_YOA