I believe his words were something to the effect of “they are still assessing”. That’s what analysts do. They will assess and come up with initial conclusions, and then they will be asked to assess further.
As I have posted, the analysts are asked to “get it right”, especially when lives are at stake, and if you have ever worked problems like this you would know that you can almost never prove anything with 100% certainty, so what the analysts are asked to do is provide an estimate and an uncertainty on that estimate.
And General Mattis is not out of line to ask for further assessment or more information, as he is the one who will have to make final recommendations to POTUS. Along with this, the evidence will have to be shared with allies (or even Russia) in such a way as to not compromise the sources or methods that collected the evidence. So scrubbing that data takes some time as well.
This is a process. The public, that doesn’t even get taught basic civics much less how these functions work, gets all wrapped up in scare tactics and emotion. The cooler heads will stick with the process.
You posted a great outline on what is going on.
Which make we wonder why you asserted Russia conceded Assad’s guilt.