Michelle Obama noted that the signers of the Declaration of Independence were not "born American

On 18 June 2014, First Lady Michelle Obama attended a ceremony at the National Archives rotunda in Washington, D.C., in which fifty foreign-born immigrants were sworn in as American citizens, and she used the occasion to speak on the subject of immigration reform:

Michelle Obama turned a flag-waving swearing-in ceremony for 50 new American citizens into a platform to call for action on the long-bogged-down issue of immigration.

"In many ways, it is because of — not in spite of — our immigrant population that we grow stronger every single day," the first lady told the newest Americans-by-choice.

Fifty people from 44 nations, from Afghanistan to Zimbabwe, took the oath in the historic setting of the National Archives rotunda, where originals of the U.S. Constitution, Declaration of Independence and Bill of Rights are displayed.

The new citizens waved small U.S. flags afterward, and each got a hug from the first lady.

Her remarks at that event included the passage referenced above, in which she noted that the Founding Fathers who signed the Declaration of Independence — like current immigrants to the U.S. — became Americans rather than being born Americans:

It’s amazing that just a few feet from here where I’m standing are the signatures of the 56 Founders who put their names on a Declaration that changed the course of history. And like the 50 of you, none of them were born American — they became American.

Just like you’re about to pledge allegiance to our flag, they pledged their lives, their fortunes, and their sacred honor to this extraordinary idea that we now know as America — the notion that we are all created equal, endowed with fundamental rights and freedoms that no one can ever take away from us.

People come from across the globe to see these documents, to read the names and signatures of the very first “Americans-by-choice,” because they know that this room holds the first chapter of our shared history. And today, it holds the next chapter of our story, as well.

In a strictly literal sense none of the Founding Fathers was born in the United States of America, because that country did not exist until they brought it into being through their efforts. It is true that many of the Founding Fathers were born in the original thirteen British colonies that  
later became the United States of America after those colonies jointly declared their independence from Great Britain in 1776, but Michelle Obama's remarks did not, as implied in the example quoted above, express her ignorance of that fact. 

What the First Lady was communicating on this occasion was that the Founding Fathers were not born into a fully formed and established America with its own history, customs, culture, and values, as modern American children are; they were born into a very different world as British subjects in a colonial empire, and they chose to seek new opportunities and lives for themselves by transforming their world into something distinctive (through the establishment of a new nation) — just as modern immigrants born outside America choose to transform their worlds by opting to leave their homelands for the United States and seek new lives through becoming Americans