Honestly, it wouldn't be surprising. A LOT of manga authors purposefully tank their series when publishers intervene. A notable example is Food Wars. There was apparently publisher interference, so he purposefully made a dude with super powers, brainwashing, this weird incest plot and a completely, utterly trash ending hated by all.
Another instance of interference was High School DxD. The reason why the MC never gets laid even though everyone wants to fuck him is because the publisher basically vetoed any fucking and kept it as pure Ecchi. He didn't drive the series into the ground, but since there was no sexy times, he basically ditches the ecchi stuff later on and it basically becomes another shonen title. It was kind of absurd.
So it DOES make sense that if Isayama wanted to do a different manga, but his publisher was adamant about AoT, he just went 'fuck you' and tanked it out of spite, since he knew it would sell.