Good point. If I retrofit the term "resolution" in place of "velocity", the author's statement does make a lot more sense. If that's what was intended they really should have used that terminology, because I was similarly confused at the term "velocity."
Whenever I've worked with time series data I've always referred to the granularity of the time dimension as its "resolution" - this is also common in geospatial data. I don't think (but am happy to be corrected) that "velocity" is a term of art in timeseries analysis.
Ya I was confused by it at first too, it definitely could have been more clear. But it's also been my experience with time series that more resolution isn't always better.
My guess is that he's referring to the resolution of a time series. E.g. Going from months to weeks is 'more' data, but makes your models worse.