Yes, the vanilla spawn rates for thralls are in some spots are less than 1%, however, these spots can spawn different thralls so the chances that there is ANY named thrall is higher than 1%.
Some spots have a higher chance to spawn a thrall, for example the Cimmerian Berserker who has a 4% chance in some spots or a 100% chance at a specific one.
That said, getting one specific thrall can take a very long time, especially if it’s one that is sharing its spawn point with others like many armor / weapon smiths and carpenters do (and as I said, the chance that there is a named at all is already low and then you have to be additionally lucky to get the specific one out of 7 others that can spawn there as well).
But all of those low chances get somewhat offset by the fact that clans can actively farm several spots with several people, it will still take quiet some time but it’s obviously a lot faster than doing it alone, so for single player, it can be just frustrating to farm ages and not get the thrall you want, which is why I personally installed the mod that mildly increases the spawn rate for named thralls (so that they are still rare but it doesn’t feel like a boring grind anymore).