Well. I have the same problem. mid 2015 macbook. It was mine, i gave to my parents, I visit them later to see it with a completely fully swollen battery. I do the battery swap and it's been slow this whole time. Only after a while did I realize it is suffering from the 800mhz problem.
I have no doubt the swelling may have damaged something however visually it is very clean and mint looking inside. I will run the diagnostic and see if I get the same one and then I do have a multimeter and can possibly check that line. Don't think I have any resistors on hand though... and dunno if i have bandwidth to do any soldering.