Yes, you are right.
But the Notion sounds do get up to 25000Hz (from Niquist) when measured, so that's just slightly higher than 48KHz (24KHz Niquist) on the sound card.
The only problem is that it adds a 'dither bit' (on output) with the Notion samples actually being at 16 bit. The 'dither bit' is also stretched compared to 'bit perfect' 20/24 bit dithering.
On top of this, Notion 6 'cuts off' high quality VST plugins (which may reach 96KHz 24bit) and 'reduces' available headroom in the process to 48KHz 16bit (24bit dither-pad).
This approach is similar to RealTracks engine; your VST's may be the best in the world but the Notion engine still 'cuts off' all higher bits and frequencies. Essentially putting a 'dither wall' between your composition and the sound quality you could have attained.
Solution:
A way around this is to render the Notion instruments in Notion and the (MIDI) VST's in a high quality VST host; then apply a 'dithering reinforcement pattern' (sound reinforcement template) to 'reconstruct' the instruments of Notion and then a 'summing pattern/pipeline' on the VST-host-output and Notion-output.
The articulations are state-of-the-art and so I think Notion can get away with it.
State of the tool:
Most people don't notice the difference and 48KHz is acceptable, but many musicians and also good listeners may observe the difference.