Their point is that there are dozens of instruments that also fit that criterion; i.e. the human voice is not the only instrument with that capacity.

Fretless sting instruments allow for continuous pitch modulation - that includes the violin family as well as others like pedal steel guitars, the Japanese shamisen, etc. Certain wind instruments like slide trombones and slide whistles do the same. There are also electronic instruments like the theremin or any synthesizer with a pitch bend wheel.