I thought that was just general "special overworld" encounters? Is serebii's strong% thing just some typos, then?Strong% just means an overworld encounter that's always in a certain spot, like the Onix you see when you enter the Wild Area for the first time. For some reason the Pokedex doesn't show where Pokemon that appear this way appear, which is why Milotic and some other Pokemon are Area Unknown.
Just as an example, that Onix at rolling fields is just an undefined % like Vespiquen, Ninjask, and so on https://serebii.net/pokearth/galar/rollingfields.shtml
But Milotic (in the fog), Lapras, & Seaking are all Strong% https://serebii.net/pokearth/galar/lakeofoutrage.shtml