Well .. it's of course possible that he's lying. It's also possible that he's been given fake intel.
For those who may not have been following this: here's what General Mattis said (from the CSPAN transctipt - ALL CAPS unfortunately).
"I AM CONFIDENT THE SYRIAN REGIME CONDUCTED -- A CHEMICAL ATTACK ON INNOCENT PEOPLE IN THIS LAST WEEK, YES, ABSOLUTELY CONFIDENT OF IT. WE HAVE THE INTELLIGENCE LEVEL OF CONFIDENCE WE NEEDED TO CONDUCT THE ATTACK."
All I can say is: if there is evidence so damning that it can change the tenor of Mattis's public statements in a single day, let it be released.
The rest of the intel came in.
If the “Assessment” the White House put out doesn’t convince you, nothing ever will. Just what source would you trust?
Incidentally, yes, I know, some will bring up Iraq. However, there is no evidence Assad is trying to “fake”* local enemies into believing he has chem weapons while telling the West he has none. That was part of Saddam’s mistake - all of the West’s (and some other) intel believed Saddam’s officers, who believed and essentially said “I don’t have it, but I think “the Guard” or unit “xyz” has it.”
*Instead of fakes, there is quite a lengthy record now of Assad’s USE of chem weapons in the last couple years. Again, see the White House Assessment. Assad apparently heeds Nike and “Just Does It”.
Also... I have not heard of any reputable claims that we hit anything other than chem weapon facilities. Maybe I missed them.