According to whom?

More accurately, some people think it is a GPL violation. There is no clear cut answer. You can get legal "opinions" either way.

My personal view is that it is NOT a GPL violation. This is implied for reading the Linux source code, which explicitly allows for non GPL kernel modules.

So my understanding is that almost all mobile makers using Android are also in the violation of GPL, since they ship blobs to support their hardware. Right?

From what I see, this is just SFC trying to leverage Oracle to change it's licence manipulating public opinion because ZFS is something the "public" wants.