No, you require Pianoteq because it's not separate from the instruments at all.
All the instruments are in every version of Pianoteq already, they're just locked in "demo" mode. That's why you can try them all. They're not plug-ins. When Modartt releases a new piano model (or models) they release a new version of the software.
When you buy an instrument you're really buying a code to unlock the instrument within Pianoteq.
Pianoteq comes with a minimum of two instrument packs. You can, if I understand the current arrangement correctly, choose any two packs when you buy the software. This used to be different - you had a limited selection of three different pairs of instrument packs. You might want to double check that.
StephenG