I trust them because they have solid track records about being accurate and meticulous about their camera testing. The only track record I have for most of the DPR tech heads is that they like to tear up Canon raw files and complain about them...A LOT. They could claim (or literally) have 10 Ph.D's...that wouldn't change their track record.
That is interesting because the DPR tech heads have a solid track record of ISO 100 DR measurements matching up with DxO.

While DPR does not.

They enjoy complaining about Canon DR, I won't fault them for doing what they enjoy...rather than getting out into the real world to enjoy photography...although I think they might be happier doing the latter. 
Some of them actually have much more extensive galleries, from around the world, than the people telling them to go out and shoot.

That is pure assumption, and will be until the camera actually hits the streets and non-beta software is used to evaluate IQ.
Maybe production models will vary appreciable from the beta cams, but that has not been the case in the past.
Beta software is not being used for the DR tests. The only place people have used beta software is peeking at high ISO performance where it seems to show the 5D3 doing better than the 5D2

.
Even testing with a pre-production sample and beta software, we do know that Canon HAS DELIVERED several improvements with their latest sensors: two stops better Native ISO for both the 1D X and 5D III;
Yes, but two stop more native ISO is different than 2 stops better SNR in RAW. Not that is a bad thing though.
Even Canon themselves don't claim 2 stops better SNR for RAW.
elimination of fixed pattern noise leaving behind more appealing random noise;
Not at low ISO, although the horizontal banding appears to be 100% gone and even the vertical doesn't appear to show up as much at high ISO (although it hasn't been carefully tested yet). But yes it does seem that the high ISO noise will look nicer, less clumped, less giant chomra-blotched, no horizontal banding at all and potentially less objectional vertical banding (at high iso only). Certainly nice improvements, no doubt.
and 6fps on the 5D III (almost double its predecessor WITH an increase in resolution, even though it was modest.)
6.0 isn't almost double 3.9 unless the D800 as almost double the MP of the 5D3 and it does not.
That said getting to 6 is big since that is the absolute minimum where it just begins to appreciably help for sports.
I wish the 6.9fps or 7.5fps rumors had been correct though since then you'd really be talking. But 6fps will sometimes get you more than one good frame, nearly as often as not, but 7-8fps almost always will. Still it is much better than 4fps which hardly ever will and it's much better than even 5fps which will only do that from time to time.
Saying Canon has not delivered even a "single" improvement over the last four years is just flat out wrong, even in the case of DR. Canon has consistently delivered improvements to DR and low ISO noise...they have just been smaller and smaller improvements as they have approached 12 stops.
When did I say they haven't offered a single improvement? I've said that the video might be much better, the fps are up, the AF should hopefully be way better, high ISO should be somewhat better.
As for DR they really haven't improved that for 4.5 years though (at base ISO).