Also linguist here. You're right that there's no right way to understand your language, but it's also the case that you need to take a speaker's self-report with a grain of salt.

That speaker has probably seen singular they countless times without raising an eyebrow, and probably used it themselves. They may report being uncomfortable with it when their attention is called to it, but that doesn't always reflect their actual usage.

You get a better understanding by analyzing a corpus than by asking people. And the corpus makes clear that a lot of people have been comfortable with singular they, for a long time. I've even seen examples of people decrying singular they in sentences using singular they.

The self-reports are also data that's informative to the linguist. But they need context to gain perspective, because they're not always identical to what the speaker generates or accepts.