The Germany goalkeeper swapped London clubs ahead of the 2022-2023 season as he ended a four-year stay at Arsenal to join fellow Premier League side Fulham.
Leno penned a three-year contract with the Cottagers in a bid to boost his hopes of featuring at the World Cup for Germany later in the year, after falling behind Aaron Ramsdale in the Arsenal pecking order.
The 30-year-old departing Emirates Stadium came as no surprise after Arsenal brought in goalkeeper Matt Turner from New England Revolution, though Leno says he was forced out the club.
"When I realised that it wasn't about performance or quality, I knew I had to go," Leno said.
"During the preparation I saw that it's not about performance, it's just about politics. It was clear to me: I have to get out of here."
Arteta appeared bemused when Leno's comments were put to him ahead of Arsenal's Europa League clash at Zurich on Thursday, claiming the suggestions were unexpected.
"I'm really surprised and I don't know if he's talking about the politics when he was starting every match or when he wasn't playing," the Arsenal manager said. "Really surprised about it."
As Leno looks to settle into life at Craven Cottage, Arteta looks ahead to a European campaign with Arsenal, who have been drawn in a group with Zurich, PSV and Bodo/Glimt.
The Gunners are among the pre-tournament favourites to go far in the Europa League, alongside their Premier League exploits that have seen them win five of their opening six games.
A 3-1 defeat at Manchester United ended the perfect start for Arsenal, who remain top of the league by a point, but Arteta knows he must balance players' workloads – especially Bukayo Saka.
"We will try to do our best to manage [Saka's] load, understanding that after November we have something that we've never experienced before," the Spaniard added, referencing the World Cup later in the year.
While Arteta may have to be cautious with star winger Saka, competing on the European front will offer other players a chance such as recent arrival Fabio Vieira, who signed from Porto in the transfer window.
"Hopefully you're going to see a lot more of him in the next few weeks and I'm sure you're going to enjoy," Arteta said of Vieira, before confirming the Gunners will be without Emile Smith Rowe due to injury.
"Emile felt some discomfort again in an area that he's been dealing with now for a few weeks. He hasn't trained in the last few days so he's out of the squad."