No one is saying that. They signed him, because he was a good player, who happened to be available on a free. Again, the same is true for Kamara. You made the point, that Liverpool went ouf of their way to sign van Dijk, as a player they actually wanted over other players available on a free, since he was always going to be better than those players. Again, nobody would argue against that. The whole argument is that Matip has been a very good player for them, who they got on a free and who made signings like van Dijk and Alisson easier, since they are also operating on a limited budget. Which makes bringing up Matip in conjunction with van Dijk relevant, especially since he's been van Dijk's main partner in defence for them.
You and I have no idea whether Kamara actually is a player we want or don't want. Kamara would hardly stand in the way of us signing de Jong, Tchouaméni or other players.