What a pile of BS. Even the rebel-friendly Syrian Observatory reported that at least 65 missiles were shot down - and of course the media failed to report that, even though they usually quote them on everything.
http://www.syriahr.com/en/?p=89324
Mattis is lying through his teeth that none of the missiles were shot down, but at least he was truthful enough to admit back in February that the US has no evidence for any gas attacks by Assad:
http://www.newsweek.com/now-mattis-admits-there-was-no-evidence-assad-using-poison-gas-his-people-801542