The idea that Jiraiya can 1v2 Itachi + Kisame is asinine. Either these two grossly overestimated Jiraiya, grossly underestimated each other, Itachi was lying in order to avoid killing one of the leaf's strongest assets, or we chalk it up to part 1 stupidity. Regardless, Jiraiya would get decimated in this fight and here's why:

General Scaling

It's safe to say that a Jiraiya starting the fight in base should be ROUGHLY equal to Orochimaru. The same Orochimaru that got borderline fodderized by an injured, exhausted, terminally ill, literally seconds away from death Itachi. Add Kisame, who is in the same ballpark as Bee (yes it was a favorable matchup for Kisame but the point remains they are in the same tier of strength) who in base kinda humiliated MS Sasuke + Taka, and this becomes unquestionably one-sided to the point of it being foolish to make a case for Jiraiya based on anything other than the above scan that can realistically be read as "too bad the person watching over Naruto is one of the Sannin. We're just some random Uchiha and a member of the 7 swordsmen. Truly no match for the likes of Tsunade".

"But Jiraiya gave Pain a good fight so he's clearly far above Orochimaru. Why are you even comparing them you fool?"

He didn't give Pain a good fight. He fought Human path which is only a mid-high jonin level taijutsu user in all of its combat showings, Preta path which is purely defensive, and Animal path which is the 2nd or 3rd strongest overall but still FAR below Deva and with only 2 summons put heavy pressure on base Jiraiya until he was just barely able to enter sage mode. The second the other paths showed up Jiraiya lost, and Deva didn't even use its abilities. Imagine a half-serious Deva vs Jiraiya starting in base. Jiraiya gets pasted no debate. At best you can scale Jiraiya's speed to Pain but that still doesn't put him comfortably above Itachi or Kisame let alone allow him a chance at blitzing.

"Orochimaru's arms were sealed at that point; it's not fair to scale Jiraiya to him."

Yes and no. He didn't have access to his full array of ninjutsu since his fight with Hiruzen, but he got around the seal by regularly swapping bodies. He'd be relatively normal for a time then deteriorate again due to the seal. During his normal window he could still use basically everything that sets Orochimaru apart from a regular Jonin (summoning, regen, etc.) with the exception of edo tensei which were extremely weak compared to the edo tensei of war arc anyway. Based on his fight with KN4 Naruto (who nearly killed Jiraiya btw) in which his body was just starting to deteriorate it's not unreasonable to say he could perform similarly to Jiraiya against those weaker paths of Pain even in his "sealed" state.

A More In-Depth Analysis

To further illustrate how bad a matchup this is for Jiraiya, let's start by looking at how Itachi alone can defeat him without much difficulty:

Genjutsu

Jiraiya has worse chakra control than Naruto, proven by his imperfect sage mode that he's unable to maintain by himself, so it stands to reason that he'd be unable to break any genjutsu Naruto failed to. Even if we give him the benefit of the doubt and say he's smart and experienced enough to never look Itachi in the eyes, what stops him from falling under finger genjutsu? Nothing that I know of.

Amaterasu

Hard counters anything Jiraiya can summon including the sage toads, and has a good chance of 1 shotting Jiraiya himself. Dodging it takes ~Raikage level speed and he certainly won't be able to pull out his little scroll to seal it mid-combat, so about the only option he has it to hope it lands on his hair so he can cut it off.

Susanoo

Negs anything Jiraiya throws at it including sage mode attacks. The only conceivable counter is sage toad genjutsu, but we've seen Itachi effortlessly reverse genjutsu from a Jonin genjutsu specialist. Is Gamarinsho probably much stronger than Kurenai's genjutsu? Sure. Though it's still featless against anyone with a single impressive defense showing so to claim it will 100% work on Itachi is questionable at best.

On the other hand, if susanoo touches Jiraiya he dies instantly whether it be from getting crushed or sealed with Totsuka.

The Addition of Kisame

His speed and taijutsu alone, which rivaled Guy up to 6th gate, should provide more than enough pressure to eliminate any chance of Jiraiya having time to summon the sage toads when combined with Itachi's offense. He also brings:

• More counters to Jiraiya's summons
  • Water Prison Shark Dance (probably solos the entire fight with this honestly)
  • 1000 Sharks
  • Water clones
• More counters to all of Jiraiya's ninjutsu
  • Samehada
  • Super Shark Bomb
  • Exploding Water Wave

So it should be pretty clear that Jiraiya gets smoked here. This is probably the most egregious quote in the series that's actually taken seriously by many people, though it's a good example of why other "x > y" type quotes from characters shouldn't be believed at face value. Itachi would not beat Obito and Momoshiki would not beat Kaguya, for example, but those are discussions for different threads.