Does it mean they don't believe that Jesus really performed miracles or does it mean something else?
I'm not a Lutheran myself, but I think it would be fair to say that ELCA contains within it people and congregations which occupy a wide amount of space from each other theologically. In other words, not everyone repeats the same thing like a robot.
Yet, at the same time, it still represents a coherent tradition and overall theological gist.
When people are said not to take the bible literally, it can mean something as simple as understanding the books of the bible as they would have been understood by people of their time, which is to say not all as literal history, but as stories meant to convey a moral meaning that may have some historical elements.
For example, the Book of Genesis contains two accounts of creation that differ from each other. Seriously. Read the first 4 or 5 chapters of Genesis (Actually, I think it'd be less than that, but I don't remember where they go from creation story #2 to the Garden of Eden- maybe it's chapter 3). That's clearly not intended to be a science textbook. It's not intended to be a literal history in the modern sense of the word. It's an origin story intended to convey a moral point- truth, but not truth in a literalistic way, truth in a more figurative abstract way.
We see in the New Testament, Jesus, who is traditionally considered God by Christians, speaking in parables- stories that are not literal histories (Are you a mustard seed or a mustard tree? Literally? The plant? Personally, I'm a mammal.), but convey a moral point.
Evolution is scientific fact. It happened. That doesn't mean Genesis isn't true in describing the basic imperfection of the human race and a strained relationship with a higher power that loves us, though.
The impression I get is that most ELCA Lutherans are not literalists simply in the sense that they understand the Old Testament as it would have been originally written- not a strict history, but stories that contain some historical elements stretched and rearranged or reshaped with some ahistorical elements to make moral points.
However, I do get the sense that the vast majority of ELCA Lutherans literally believe in the miracles of Jesus. There are are likely some who don't- that's part of that theological diversity I mentioned, but every ELCA service I've been to (Which, granted, isn't many, I'm not Lutheran) has seemed to speak of Jesus as a real historical figure who did miraculous things.