The Gemara has a Hava Amina that only a wooden utensil is secondary to its plating if the plating is "standing" and the Shuchon's plating was not "standing." Therore we consider the Shulchon as a wooden utensil.
Rashi explains "standing" as if you nailed it in well to the utensil. Tosfos explains that they didn't attach the plating to the Shulchon. Tosfos explains that they had two gold molds that surrounded the wooden Shulchon. This is similar to the Aron that was a wooden box between two gold ones.
Tosfos gives another explanation to "standing" that the platingwas thick and heavy that can stand by itself without the wood holding it up. That only if it was plated by such a material do we say that the wood is secondary to it. This is similar to the case of the Braisa of the marble plated table that the marble is free standing. However, the Shulchon's plating was too thin to stand by itself so we should say that the plating is secondary to the wood
The Tosfos project
tosfos.ecwid.com
Tosfos in English. Still free copies available.