I think it's because TimescaleDB has, for performance reasons, "hypertables", and these functions operate on those.
Yep! Though I will say a number of the hyperfunctions are optimized to work on time-series data whether or not there's a hypertable involved, but the name is meant to echo the hypertable concept. In general, hyperfunctions are meant to improve the user experience for working with time-series data inside of SQL.