So I started worrying a bit, thinking that maybe Marianski was only talking about hard pork fat instead of hard fat in general. So I researched a bit more, and found this video of Aaron Franklin trimming a brisket. At around 2 mins in, he points to the soft top fat on the flat and says it's bad for sausage and then points to the deckle fat and says it's good for sausage. He then repeats that about 11 mins in, pointing to the hard fat on the top of the flat and the deckle fat.
So I think that because there's a lot of "bad fat" for sausages in the brisket, it gets a bad rap for use in sausage making. But the key is not to avoid brisket sausages, it is to rather trim the bad fat and use the good fat.
So I'll definitely be using the deckle fat from now on, and using that to replace the fat I'll now be trimming from my slices from the flat. Just posting here for posterity.