If the water column is sufficiently fertilezed, than yes it doesn't need substrate.. Tho substrate can be a lot, for example from the lfs you get it on rockwool, i've growen this plant on filter sponge in the sump, it can hardly be called substrate, but its called a good "grow medium". Thus there can be used different types of medium to give the plant the ability to anchor its roots in or around. Think of mosses and or even other epiphytic plants like anubias. Than the Anubias attaches as epiphyte to the wood and the H. tripartita can use this to tangle its roots around.
Substrate or grow medium can be a very versatile, very porous piece of lava rock can be a perfect medium to grow plants to. Example when it's crushed and lays on the bottom than suddenly it's a substrate, if it's in one big chunck it becommes a rock again. But still as a bigger chunck partialy it has the same properties, the plant can still grows its hair roots into the rocks porousity. Doesn't make the plant an epiphyte, but the growing concept is simmular.
We need to make sure the plant gets what it needs over the water column if the medium it grows to doesn't have that.