It's embarrassing that the people on the correct side are chasing their tails on definitions and reference frames. Brercrow must be (justifiably) having a good laugh.
Yes, if you consider the aircraft a point object (like a ball) without a heading, then groundspeedvelocity can only have a direction and positive magnitude. But it's slightly more complicated than that, in that an aircraft has a set of body axes and everything is normally calculated relative to them as the direction component of the reference frame.
Again, before anyone got lost in the weeds of this thread, everyone would agree without a moment's hesitation of the trivial truth that in my scenario A, groundspeed is 90 initially and then 110. Not negative 110.
And again, imagine a dead calm day with no wind. Then, everything that is true about airspeed is true about groundspeed. If groundspeed is only positive, then airspeed is always positive, and therefore airplane should fly just the same in a tailslide as they do going forward. True or untrue?