Well Terry Matallis isn't the TNG Cannon Guru he thinks he is. Shaw also brings up the temporal anomaly in the devron system from TNG S7 All Good Things, but at the end of the episode itself, John-Luc makes a log entry stating that nothing in the reality he experienced happened in the 'real world'. He said he was the only one with any memory of what happened in any of the three Q created timelines..
There's also the fact that on DS9 Odo informed everyone that changelings perfectly mimic the objects that they are in at the time. "If you scan me when I'm a rock, you'll read a rock."
In DS9 S3 The Adversary, Sisko & Co find out that an ambassador who sent them on a mission is in fact a changeling; and Dr Bashir States that when he did a medical scan of that ambassador, the Ambassador showed as completely human.
So yeah changelings being able to fool Federation scanners is nothing new. That said maintaining their last form after death is.