Shrug, you can call it an apple I'll call it an orange. At the end of the day it doesn't matter since the game is the same game and whether it is megaten or not-megaten won't make a lick of a difference.
I just find the megaten umbrella pretty wide these days.
What makes megaten, megaten?
Is it summoning demons? DDS Avatar Tuner are megaten games for sure, but you don't summon demons.
Is it using megaten spells like agi? What if you make a non-rpg megaten like a visual novel? Now it's not megaten because you aren't using the spells?
Is it that the game borrows from demon mythology? Because there's been plenty of speculation Catherine features at least one if not more demons from mythology
etc..etc...
I don't think there's a quick and easy way to define what constitutes megaten spinoffs anymore. Take Catherine for example. If at some point in the game you meet the P4 characters or you find out the game takes place in the P4 world, would that suddenly make it megaten because it's a spinoff of a spinoff? Or would you just call it a cameo and say its not megaten. Or if there was a mini-game where you went to the demon world and could talk with Jack Frost does that suddenly make it megaten? Or would that just be a cameo. At what point of involvement does it qualify as a megaten spinoff? It seems pretty vague to me these days.