Puri.sm is going to release a debian-based Linux smartphone called the Librem 5 in a few months, and users will probably be looking for a good note-taking app. Someone might be able to compile Turtl from source for ARM and use that, but would be stuck with a desktop app on a tiny phone screen.
I don’t think it would be very hard for Turtl developers to support the Linux phones. Just take the interface from the Android app, marry it to the existing Linux version, and recompile for ARM processors. (It might be simpler than that since Android is sort of running Linux, but my knowledge of that is limited.)
As a side note, Puri.sm is working on an App Store of Free Open-Source Software for their Debian-based PureOS. It would be amazing to see Turtl in the PureOS app store, but that might be a longer-term goal depending on what that entails.