> How is a new dictionary supposed to source definitions without using current dictionaries? Aren't existing dictionaries essentially the "canon"?

No, because that isn't how dictionaries work. Dictionaries, and this goes back to the very first dictionaries, have always paid people to look at current usage and write definitions based on that. Dictionaries are descriptive, not prescriptive; they reflect usage, not control it.

The OED is well-known for its citations backing up its definitions, and they have a very strong bias towards actual usage as opposed to other dictionaries.

Edited to add: Some dictionaries are prescriptive to an extent. Those are the ones that define terms used in a controlled vocabulary, such as technical terms used in a specific field. When the CRC handbook says that this is what 'butyl' means, for example, it is pretty well taken to be prescriptive among chemists.