I have seen most of the Pokémon series and I don't think that Meowth has ever seen a Sandslash so he has no idea which one it is and just thinks that its a big fat and spikey Sandshrew. I mean, Team Rocket isn't all that bright on naming Pokémon, around the same time Jesse and James both think that the Scyther is an Alakazam.
Yes, I agree. How would Meowth know? Even though he himself is a Pokemon, I don't think he would know all the Pokemon by memory. He's usually a wise cracker anyways, which proves with the joke when the clones broke free. "Send in the clones!"