In Scandinavia, the indoor game was at home, the Kings Cup was one of the oldest tennis competitions, always played indoors on fast carpet in Sweden or Denmark. So its not so surprising that Borg had to play indoors since his junior days. He also learned quick reflexes by playing bandy.
Indoor too has fast and slower courts. I thought that Dallas always had a quite slow indoor court, seing the long rallies in the matches between Laver, Rosewall or Borg. Also Vilas did win Dallas one year, and a grinder like Eddie Dibbs did pretty well at Dallas too. The Nadal poor indoor showing has imo two sides: Maybe he is lucky, that carpet is gone, maybe he would play better, if he had more experience und time to build up his game on fast indoor courts.