Even if you can see the triangle move a little in your video, looking carefully, this is totally overwhelmed by the excessive flex of your wheel (do you have play in your wheel axle?). Anyway, the triangle will always flex, bikes are not made of concrete. Mine does, but not more than my Altitude, and the feeling on the trail is fine, for the moment.
There is no movement in the axle.
Yes the wheel flexes but no more than any other wheel I have.
Tried it with two other wheels and the triangle, specifically the lateral movement of the seat stays, is the same.
Also had it in the KICKR with the same movement. I would record that but it is on a rocker platform and I don't want that movement to confuse you like the tire...
Only one other carbon FS on hand at the moment, it flexes but about 10mm total. Different platform and not a relevant comparison.
I know they flex, it is part of the design. How much flex is too much flex in comparison to the the same frame has always been the question. Since you're having trouble discerning the difference between the tire and triangle movement it is about 20mm of seat stay flex. It was more without the shims and now it feels fine.
How much does YOUR triangle flex? "Not that much" isn't helpful, I'm sure you can understand that.