Chunk encryption is done symmetrically, not asymetrically.
In symmetric encryption both encryption and decryption are done by the same key.
De-deplucation works network wide, because regardless who encrypts a file, the resulting encrypted chunks always end up the same if the source file is the same. It’s the principle behind self-encryption. The datamap contains the keys with which the chunks were encrypted, and because it is symmetric encryption they can also be used for decryption.
The difference between a public and a private file is that the public file’s datamap is uploaded to the network without encryption, and for private data you encrypt the datamap with your public key (so you can decrypt it with your private key. Here we do use asymetric encryption).