So, it is not true that weed still grows in a fixed joint. If it does, then errors were made during installation and the joint was not completely filled; for example, if you leave half a centimeter open at the top, dirt, soil, seeds accumulate there, and then it grows.
It can only be the case that with paving stones with a rounded surface, such as the Nordland paving stones from Berding Beton, made in the style of an old village street, dirt collects in the large rounded chamfers between the stones, which preferably leads to moss growth if you do not sweep the surface. But since you have a fixed joint, cleaning is very easy, whether with a street sweeper, broom, or high-pressure cleaner. For example, I myself have some moss between the stones of this type in front of my carport due to standing moisture, only shady location, and a lot of dirt from the trees. See the photo of the gray/anthracite in the attachment, taken back then after installation, where you can clearly see the rounded edges.
But with normal sharp-edged paving stones, with normal or micro chamfers, not enough dirt remains to allow weed to grow there at all—see the photo of my terrace with the yellow/beige polygonal concrete paving stones, where the joint is slightly deeper than the stone.
In that case, it can only be that the substructure is too weak, causing the stones to sink and the fixed joint to break open; then weed naturally grows through quite clearly.
By the way, DanSand has become known for joint sand, stone meal, etc., but these were only dry infill products; I have not yet experienced their joint mortar myself.