Over my career as a debater, I've often seen this feat cited in order to lowball General Grievous to being below even a standard Jedi Master, or hype up Kit Fisto to rivalling Obi-Wan Kenobi. Both claims are obviously ludicrous. While it is an undoubtably impressive feat for Kit Fisto, it does have several circumstances.

Before I get into this, first let's review the video of their final fight in the episode Lair of Grievous:

Now from this we see how the fight goes:

• Kit Fisto is initially driven back, and is forced to avoid confronting Grievous directly
• By exploiting the environment Kit Fisto confuses Grievous
• After dodging a single blow, Kit disarms Grievous of one of his blades
• Only after disarming him does Fisto actually fight Grievous head-on
• They fight evenly until Kit uses telekinesis, then Grievous gets driven back
• MagnaGuards arrive before anything else can happen

Now before I get into analyzing this fight any further, I should note that Grievous wasn't completely repaired. His medical droid notes that he's "in no condition to fight" and on top of that, the novelization of the episode actually backs this up, noting that black smoke was pouring out of his armor:

The general quickly leaped up, black smoke pouring from his armor.

The Clone Wars: Grievous Attacks!

Now onto the fight itself: as is made evident from the video, Grievous seems to be driving Fisto back, forcing him to use the environment. This is confirmed by StarWars.com saying Grievous overwhelmed Kit Fisto:

While Gor was destroyed, Grievous himself killed a young Padawan called Nahdar and overwhelmed Kit Fisto in lightsaber combat. The General survived to fight another day.

StarWars.com: General Grievous

And the novelisation further backs this up:

Kit flew out of the fog and engaged Grievous. The general spun his four lightsabers like propellers and drove Kit back.

Suddenly Kit somersaulted over Grievous' head. The general turned and slashed with his lightsabers, but Kit had vanished back into the fog.

The Clone Wars: Grievous Attacks!

It'd make sense for Grievous to overwhelm Fisto in direct 1v1 combat. He's already pushed back both Kit Fisto and Nahdar simultaneously, despite fighting in a disadvantageous environment where he couldn't maneuver properly:

He immediately ignited his two lightsabers and swung them at the Jedi, but the walls of the corridor were too narrow for him to maneuver.

[...]

He turned his body side-ways so that he could fight the Jedi on either side of him with two lightsabers each. It took all that the two Jedi had to hold off Grievous' advances.

The Clone Wars: Grievous Attacks!

What we see here is that a fully-repaired Grievous, despite an environmental disadvantage holds an advantage over both Kit Fisto and Nahdar Vebb, and it took all that the two combined could do in order to hold the General off. Yet the damaged Grievous gets stone-walled by Nadhar... This should speak to how crippling his injuries were.

Next up: Kit Fisto disarming Grievous. Let's take a bit to discuss why this isn't that impressive... Grievous, in-canon, doesn't seem to be as technically skilled as his EU counterpart, and Filoni seems to have him written as an expert killer solely for his unorthodox. With that, it's been fairly easy to work past a few swinging propeller strikes and disarm the General. Heck, Depa Billaba does it:

The result? She gets severely wounded, then later in the fight we see she gets wounded by GG and he's on the winning side of the battle:

Gallery

He gets disarmed by Kenobi, then proceeds to drive Obi-Wan back in a bladelock:

https://youtu.be/Xj49nsG4ruo?t=65

Disarming Grievous clearly doesn't phase him that much, unless you slice off all four of his limbs, but doing that is nearly impossible. So Fisto disarming his isn't that big a knock against GG.

And only when Kit starts using Jar'Kai does he gain any advantage over Grievous, and even then the episode seems to have them more-or-less fighting evenly until Kit uses the Force.

So at the end of the day, it's made pretty clear that without the favourable circumstances, Fisto would get defeated by General Grievous, rather than the converse.