Technically, 1776 was an insurrection.
Well that’s a distinction without any real difference in the case of 1776.
Insurrections are a revolt against the civil authority, usually with the intent of seizing control of the gov’t. The Colonials weren’t interested in seizing control of King George’s gov’t, they just intended to separate from it. And separation is the definition of secession.
The British consider it a mutiny.