It is somewhat similar but then also quite different.
Akonadi's server access protocol is similar to IMAP in terms of syntax and
grammar. Contents of commands and responses can be different and there are
additional commands for things usually not found in IMAP.
As far as I can tell there is also a different usage pattern for the
database, i.e. permanent storage vs. transient storage (Akonadi uses its
database mainly for caching, potentially only of parts of items, e.g.
envelope of mails).