Are you saying that the two are synonymous?
They declared their intent to secede from the political union which bound them to the Crown based, not on any legal right recognized within the applicable courts, but rather, on the natural right of self-determination from which all developed political power (theoretically) flows.
How could the colonies have been in a 'political union' with the Crown when they were denied any representation in the legislative body?
The above question being evidence of an unfortunate combination of binary thinking and poor reading comprehension, it falls to me to point out that secession and rebellion are obviously not mutually exclusive. Imagine, if you can, that there was once a controlling authority which did not sanction withdrawl from its influence. In fact, it expressly forbade it! To secede in spite of a directive to not do so would be, necessarily, an act of rebellion against the issuing authority.
How could the colonies have been in a 'political union' with the Crown when they were denied any representation in the legislative body?
The same way most people in this world today are in some political union without representation. The same way the conquered states were forced back into a political union actually called "The Union" during reconstruction without representation. The history of the world beyond your echo chamber is full of examples of non-represenative governments controlling people.