>> The website said the variety was Artemisia dracunculus "Sativa". So, happy gardener here googled that up to have a look see.
>> Found out that Sativa doesn't produce viable seed.
I don't know whether this applies to the case you described, but I think that seeds are often sold of plants that "don't come true from seed". That would be true of any F1 hybrid variety.
There are probably other hybrids that don't produce any seed, or only produce sterile seeds.
Yet a vendor could still sell seeds of that variety - the product of crosses between two parental strains, each of which is OP (breeds true). They produce salable F1 seed that is viable, that reliably produces the named F1 hybrid cultivar. However, that F1 plant can't breed true with itself, or might produce only sterile seeds or no seeds at all.
Does that sound right?
Could it be relevant to what you described?
