It is indeed possible that Chinese trading vessels may have reached the Kamchatka peninsula and the Aleutian Islands. The more unlikely part here is that those voyages would have been made known to a random Venetian barbarian who happened to be passing through, and that said Venetian would be able to obtain such detailed maps. If I were going to come up with a hypothetical explanation for the maps beyond 20th century forgery, I'd say that it's more likely that some 16th century Manila galleon strayed a bit north on the return trip to Acapulco and did some secret mapping. The Spanish Manila route actually got surprisingly close to Alaska: http://historicphilippines.com/wp-content/uploads/2014/05/ru...

That's a stretch too, of course. But the Spanish were extremely secretive about their cartographic knowledge so the mere fact that there was no official announcement of a discovery doesn't necessarily mean they didn't know about it.