The Adventist Church, by way of Ellen G. White in The Great Controversy, have latched onto the idea that the proto-Protestant Waldensians were staunch seventh-day sabbatarians but were then eradicated by the Papacy—another falsehood. They are still active in Italy and Argentina-Uruguay.
The Adventist Church tries to assert this because they think it helps bolster their teaching and show it’s historicity. They claim the Waldensians were guardians of the “sabbath truth” amidst Papal persecution.
The Waldensian Churches are Reformed and align themselves with the Presbyterian Churches. In Latin, they were called Mater Reformationis—which means Mother of the Reformation. This is because they were around before the Middle Ages, but were not a formal church. In 1559, they adopted the Huguenot Confession of faith called De la Rochelle. In 1655 they formulated their own confession which was quickly drafted in Italian immediately after their massacre called Piedmonts Easters. This was a shortened version in Italian of the 1559 Huguenot Confession of faith—which confirms that the theology of the Waldenses aligns with Presbyterian Calvinism. This is still the basis of the Waldensian faith today and ministers in their church’s have to agree to uphold it. They don’t observe the seventh-day sabbath today just like they didn’t previously.
They are not (and were not) seventh-day sabbatarians like the SDA Church claims. They affirm the first day to be the Christian sabbath and their usage of the word “sabbath” is in reference to this. It’s quite ironic that the Adventist Church is staunchly anti-Calvinist and yet they appeal to a Calvinist sect to try and bolster their central, foundational claim. And why is it only this doctrine? Afterall, the Waldensians look nothing like Adventist theology. Their confession of faith can be read here.
The Ellen White Estate has even recognized the issues with this claim by quietly updating the appendix of the Great Controversy.