I've never known sunflower seeds to fall into the nuts category.
It's a confusing subject because there is a culinary distinction similar to that of botanical fruit & culinary vegetables/culinary fruit that occurs with nut/drupes/ pits/seeds/hips and so on.
The definition of a botanical nut is
That bit is easy.
The next bit is harder.
The definition of a drupe is
With drupes, we eat one of 2 parts, either the fleshy body or the inner seed.
for example fleshy body - peaches, nectarines, plums, cherries, dates, mangoes and also blackberries and raspberries, (but not apples, pears, quince and so on because their seeds are not encased in a hard shell/stone.)
However another example of drupes where we eat the inner seed are walnuts, pistachios, cashews, macadamia nuts, almonds, brazil nuts, coconuts and pecans.
Seeds fall into a seperate category completely because they don't have the hard shell surrounding the seed or the fleshy part, eg. pine nuts, sunflower seeds, poppy seeds, pumpkin seeds, flaxseed, and sesame seeds...
