My understanding is that the ADS-B data originated from a RadarBox, not an SBS.
This means that we can't be sure whether the altitude readouts were relative to 1013mb, or were corrected for the prevailing QNH (RadarBox will do the latter automatically, but only if the appropriate box is ticked).
Yep, in the 2009 version of the software, you enter the transition altitude and QNH. Since the QNH must be entered manually and the online RadarBox's usually run 24/7, most users leave 1013 in the QNH in my experience. But, as you correctly observe, we can't be sure without more information.