I have heard the same thing several times...Dem People said:DO NOT USE the SIG SWAT flash hider with an AAC can. AAC told me at SHOT one year that they were originally working with SIG, but SIG told them to bounce and just made a poor copy of the hider. AAC will NOT warranty any issues using the SIG hider.
PS the M4-2000 will still screw to the hider, but I have never tried it. Sold off my SWAT hider.