It should be Tämä pirtelö on herkullinen, not herkullista!
And it's inconsistent.
Before was sentence with "marja on herkulinen", and here "pitrelö on herkullista".
There is no clear indication in this sentence why I should use partitiivi, but not nominatiivi.
Is it ‘herkullista’ because I am only tasting part of it ? If the whole unit (glass) of milkshake is delicious would that be ‘herkullinen’?
THIS milkshake is HERKULLINEN.
Milkshake (in general) is HERKULLISTA.
Isn't that the way it goes?