I believe I once spoke to some NPC that discussed this topic... but I don't remember exactly... I believe it was something with being solid in a different way; I think a property unique to notum/novictum was mentioned too...
I guess people interpret things differently when things are only told partly. You might be right about Roman's immortality being his own accomplishment (though the mystical visitors often mention "great gifts" given to him to accomplish the mission he was given, while all they did was giving him a hand in getting out of Russia...
But I'm pretty sure David really experienced the travel between the stars completely. When you read it you really get the feeling of some endless travel; it seems to drive him crazy; a more clear clue is "the books he already had read 1000's of times" for example.