I guess it comes down to what is causing the flecking. If the flecks are nubs in the yarn as a result of the wool being washed and felted before being spun into it, the tweed is then a Donegal-style tweed, regardless of where it's from. For example, I have a "Donegal" tweed jacket made from Porter & Harding cloth, but I have no idea where it's actually from. It's just called Donegal because of the sort of flecks it has.
So, it seems to me, if Harris tweed is defined by both the yarn type and place of origin, and such yarn cannot be spun to have such nubs, than a so-nubbed tweed from Harris is a Donegal tweed and not a Harris tweed, even if it is from Harris. If the yarn can be spun in Harris to qualify as Harris tweed yarn and have those nubs, then you have a Harris Donegal tweed.