The Guardians are technically human, but they aren’t dreaming; they’re physically present in Dreamland. Hence they can only fly by use of their technology/magic (i.e. wings).

The tablets Alex, Nastajia, et. al. found when they took on the Kraken spoke of seven races, including the Guardians, (chapter 6 page 418) so assuming that refers to the people of Seraphopolis, at one time they must have had contact with everyone else – they were still known and participating in creating the tablets. When Nicodemus started re-writing history, they must have had to scramble to keep from getting eliminated.

Nivienne said they could peer into the world of men much as she does with the pool of auryn. Who knows if they can’t also peer into Dreamland in similar fashion? (Besides Scott, that is.) And, if they are descended from humans, is it possible their children also find themselves traveling around Dreamland in their sleep? Now that would be getting a bit recursive! Maybe not – like the old joke. “You can’t get there from here.”