Consider a feature from earlier in the keynote: the thing Notes (and Math Notes) does now where it fixes up your handwriting into a facsimile of your handwriting, with the resulting letters then acting semantically as text (snapping to a baseline grid; being reflowable; being interpretable as math equations) but still having the kind of long-distance context-dependent variations that can't be accomplished by just generating a "handwriting font" with glyph variations selected by ligature.
They didn't say that this is an "AI thing", but I can't honestly see how else you'd do it other than by fine-tuning a vision model on the user's own handwriting.
They didn't say that this is an "AI thing", but I can't honestly see how else you'd do it other than by fine-tuning a vision model on the user's own handwriting.