I agree that it is not a "determination", but it is more than a "speculation".
Perhaps it's a language thing, but for me speculation is only some hand waving you do while drinking coffee with other physicist/astronomers in the cafeteria of the university. [Hi from Argentina!]
They made a model of the object as Nitrogen ice, and apparently it fits the observations and they published it in a peer review journal. [To be fair, the solar sail explanation is also published it in a peer review journal.]
What about "Scientists propose new explanation of extra-solar object 'Oumuamua an Nitrogen_2 ice fragment of an exo‐Pluto surface"
