> But chances are, almost none of those coming realises that the US's political and ideological birthplace was once part of a little-known Swedish colony known as Nya Sverige (New Sweden).
Or they think Virginia has a strong claim to be "the US's political and ideological birthplace." The author of the Declaration of Independence, Thomas Jefferson was from Virginia. The "Father of the Constitution," James Madison, was from Virginia. George Washington was from Virginia.
It's not like only one European country had colonies in the pre-United States.
> But chances are, almost none of those coming realises that [Philadelphia] was once part of a little-known Swedish colony known as Nya Sverige (New Sweden).
That's all it's trying to say in this sentence. It's not trying to say that the New Sweden colony was actually the US's political and ideological birthplace. (That's how you read it, right? I'm not sure how your last sentence makes sense otherwise; Philadelphia was a creation of the same European country as Virginia.)