The point is, there is no "real" Trek universe where something exists without us knowing about it. It's just a bunch of stuff people make up. The people who made up the show decided that there would not be a lieutenant commander's rank. The people who wrote the earlier ENT novels either didn't notice that quirk of the period or chose to disregard it. So now, in the novel continuity, there is a lieutenant commander's rank. And whether it existed during the series is irrelevant to me because that's not the period I'm writing about.