Oops, sorry!
GRRM did say that, but I never really believed it. He has Robert and his cumbersome entourage travel from KL to Winterfell in ~50 days which is very fast compared to, say, the travel times you see in the 1st crusade or other medieval histories where people cover a lot of distance by foot and horse. For example the frankish crusaders took ~4 months merely for the trip from Germany to Constantinople. If you imagine those kind of people walking across a continent the size of South America, I have a hard time seeing them cross half that continent in just 50 days.
Keep in mind the Crusaders had infantry. They also had an entire Army, which basically means they brought a massive traffic jam every place they went. An army like that moving 15 miles (24 km) a day is making very good time. 10 Miles (16 km) is more likely.
OTOH the King's Court are all mounted. That means 30 miles (48 km) a day would be expected, with 70 (113 km) possible on a good day.
And they're all good days. All the horses are good horses, they've got multiple remounts, they're all decent horseman, they've got good weather, and they're on an excellent road. To cross half of South America you'd have to travel about 3000 miles, which is 60 miles (96 km) a day if you've got 50 days to do it.
So mathematically speaking this trip was quite possible. Unlikely, but possible.
Nick