Sadly, Mike, you’re not the first to make that argument, which is completely VALID. Martin Luther is not the whole of Lutheranism. But many Catholics who don’t know much about Lutheranism assume as much. They ignore Melanchthon, Chemnitz, Georg Calixtus, Leonhard Hutter, Johann Gerhard, Nicolaus Hunnius, and thirty other big names in Lutheran history that shaped the Lutheran faith into what it is. Martin Luther is the high-profile spark of the Reformation and a soft, huge target. The Book of Concord, put together in 1580, was assembled long after Luther’s death. Chemnitz was the major contributor to that, which set more of the standard for orthodox Lutheranism than Luther had contributed previously. Melanchthon’s role in Lutheranism was huge and he did a lot of the heavy lifting. He also frequently was divergent in thinking from Martin Luther himself. So Lutheranism was not an ideaological monolith to be fair. It had plenty of rangling, debate, prayer, and study amidst a wife array of personalities. Luther, like I said, is the soft target of anti-Protestant folks because he was the spark, the beginning of the movement.
As you correctly point out, just because Luther believed something of individual piety or theology, if it isn’t a part of the Large or Small Catechism, Augsburg Confession, Book of Concord, and other mainstream Lutheran orthodox focal points, it isn’t then automatically gospel. I hear the argument all the time, “Luther was against birth control! For God’s sakes! Luther was opposed to it so we all must be!” They hate Luther until he agrees with 'em!