That can be more than just suspicion - it was confirmed by insiders after the announcement. The ban had apparently been in the works for some time, and the iOs action just provoked them to move up the timeline. (Which also explains the decision to release an untrained system and let real users serve as alpha testers.)
Oath apparently viewed the adult content as a nonstarter for monetization and a general threat to advertiser relationships. Since both the site's design and its massive engineering flaws meant adult content showed up anywhere, it was even driving advertisers away from not-necessarily-adult content like user feeds, and there was some concern it was even keeping user counts down.
Of course, the entire plan here was nuts. Oath apparently wanted to double user counts by summer 2019, even though growth was effectively zero and there was no plan to fix that. They confused an adult content problem with a spam problem, and so the spam is still going strong. And the monetization outlook is genuinely hilarious: Oath intended to ban adult content as a step to monetizing the site's fandom and social justice communities. Apparently nobody noticed that the fandom stuff was the porn they were banning, or that a solid 20% of all users were going to be affected.
https://www.vox.com/the-goods/2018/12/4/18126112/tumblr-porn...