That’s not a false dichotomy.
A false dichotomy would be claim that this person is either a Yankees fan or they’re lying. The dichotomy is limiting the possibility to only two options.
My statement…
…is an inference from observed behavior. I’m not saying that there are only two possibilities. I’m saying that one explanation is more probable than the other.If someone spends 10+ years defending the Yankees from criticism, I think it’s reasonable to infer they are a Yankees supporter.
An observation that a person defends something over a long period of time means that they likely have some sort of sympathy, loyalty or identification with that thing is normal inductive reasoning.
So my argument isn’t that only two possibilities exist. It’s that long-term behavior provides evidence of someone’s sympathies and loyalties. Whether that inference is true or not is different from whether it’s a false dichotomy.