You reference a map dated 1418. However, the voyages began in 1421 and return was in 1424. Therefore, the only logical conclusion if the 1418 date is correct, is that this was a map used by the 1421 explorers to further fill out their preliminary 1418 and other early information. I doubt an emperor would finance 800 or more ships based on pure speculation.
In the other camp, Menzies is supporting Liu and the 1418 map with fervor. His key reasoning, forwarded by email from a member of his staff, is that "every continent, ocean, land, island, river shown on the 1418 map also appears on other Chinese maps of the same date or earlier. There is nothing new on the 1418 map—it simply combines everything on one sheet of paper," he said.Map Fuels Debate: Did Chinese Sail to New World First?Menzies also points to a Portuguese map of the Americas dating from 1419 whose mistakes—like the drawing of California as an island—are thought to have been copied from cartographic errors made by the Chinese.
"In 1419 European voyages of exploration had not started. If the 1418 map is a forgery, then the 1419 map must be as well. How do you forge something yet to be discovered," Menzies reasoned.
It's clear to me that it's likely a later forgery.