|King Joker|
Your Excellency
If anyone is going to have an advantage in terms of familiarity it should be Vader. He's the one whose changed his fighting style and overall tactics in how he approaches combat. And, you know, he taught her pretty much everything she knows in terms of fighting when she was a Jedi. Her gaining any edge because of familiarity is seriously ridiculous. But then again, we're dealing with Filoni who isn't always spot-on when it comes to these things.
As for Ahsoka increasing so drastically, I don't understand what's so insane about it. So many Force-wielders have increased their skills without a master, proper training, etc. We don't really know anything about what Ahsoka was doing between TCW and Rebels, but I'm pretty sure there'd be plenty of opportunities to hone her skills and learn more / better combat techniques. Also, as mentioned in Rebels, there are many Jedi Temples throughout the Galaxy that weren't destroyed by the Empire, so that could've provided learning opportunities for Ahsoka if she desired to seek out these Temples.
Also, as far as Ahsoka's talent and potential goes: "While Ahsoka is gifted with raw Force talent, her tendency to follow her heart challenges her path to knighthood." - Ultimate Star Wars
I mean, ****, what non-prodigy is being offered to rejoin the Order as a Jedi Knight at 16? Please.
Any teenager who is surviving encounters with General Grievous, Asajj Ventress, and one-upping remarkably skilled warriors like Pre Vizsla is probably pretty prodigious, lol.