In Chapter 10 of Tom Sawyer, how do we know that Tom was more knowledgeable than Huck?

As the boys swear to each other that both of them will keep what they have seen and done between themselves, Huck admires Tom's "facility in writing, and the sublimity of his language," even though the reader recognizes that what Tom has written is far from sublime; it is a simple, single-sentence declaration filled with colloquial expressions, which asserts that neither of them will tell, and if they do, they will fall dead. But to Huck, it is impressively poetic.
After signing his own name, Tom also demonstrates to Huck how the letters H and F are formed in writing, suggesting that Huck is unable to write his own name.
Also, though he can't properly pronounce it, Tom understands that verdigris is toxic and tells Huck that they shouldn't use an oxidized brass lapel pin to prick their fingers for their blood oath.