Well, I didn't remember the exact quote quite right. They never say TCW Ahsoka is > Rebels Ahsoka. Instead, they imply that Rebels Ahsoka hasn't really grown much as a fighter, and has more grown in terms of fighting mentality and spirituality.
Q: How has she grown as a fighter since she left the Order in the Clone Wars?A, Henry Gilroy: She's extremely skilled. And I think over the years, she's probably had the opportunity to hone her abilities. But there hasn't been as much day-to-day ongoing battling, as she was doing in the Clone Wars. So you could say on some level, while she's become more measured, her growth is more spiritual in nature.
Both Henry Gilroy and Pablo Hidalgo basically say the same thing during this Rebels Recon (Season 2, episode 9). Her fighting style hasn't changed, but she's more in control of it. She's grown in terms of mentality and spirituality, but her actual prowess as a fighter hasn't appreciably grown, just changed gears to something more focused.
At least, that's my interpretation of what they're saying. And it makes perfect sense, too. During the Clone Wars, Ahsoka was using her lightsaber daily, in life-or-death situations on the battlefield. After Order 66, she didn't use a lightsaber again for almost a year. And even after the Ahsoka novel, she wasn't taking part in battlefield combat like she'd been doing before. She was an intel agent for a budding Rebel Alliance. She might have occasionally used it against stormtroopers, maybe, but she wasn't an openly practicing Jedi like Kanan became during Rebels. It was only after Vader learned of Ahsoka's existence in Season 2 that Ahsoka would get the opportunity to engage in another lightsaber duel. Her fight against the Inquisitors was the first lightsaber duel for her in over a decade and a half. It doesn't make sense for her to have grown considerably in lightsaber skill during that time. But spiritually, mentally? Sure, I can totally buy that she's way more capable in those regards.
So, to sum up, I'm of the opinion that Rebels Ahsoka is not more skilled that TCW Ahsoka as a combatant. Just less flashy and more focused. With the massive disparity shown during Ahsoka's fight against Grievous in Season 5, I believe Grievous would still stomp Rebels Ahsoka, with a bit more difficulty, perhaps.
Meanwhile, back on the topic at hand, DD Quinlan Vos jobbed Grievous hard. The fight was less than a minute long, and Vos almost immediately managed to disarm Grievous. This is the same Grievous who has defeated Jedi Master Depa Billaba twice, defeated Adi Gallia in seconds (mostly off-screen during one of the droid-centric episodes of TCW), defeated Obi-Wan Kenobi (and nearly defeated him multiple times), and contended well with Assaj Ventress near her prime. DD Quinlan Vos beat Dooku. I don't know what else we need to say in this thread. Rebels Ahsoka isn't beating Dooku. She's not on that level. Vos is.